European sovereign cloud spending is forecast to more than triple between 2025 and 2027. Forrester finds that 84% of decision-makers now cite digital sovereignty as a factor in vendor selection, yet fewer than half feel confident their current stack is actually compliant. The conversation has moved from political debate to operational question — and that question is: can the open-source sovereign stack actually do the job?
Two real-world deployments give us the most useful answer. Schleswig-Holstein completed migration of more than 40,000 civil servant accounts and over 100 million emails and calendar entries off Microsoft Exchange and Outlook to Open-Xchange and Thunderbird in October 2025, with 30,000 PCs migrating from Windows and Office to Linux and LibreOffice alongside it. France mandated its state-built video conferencing platform, Visio, for all 5.4 million French civil servants by 2027. Neither is a pilot. Both are production infrastructure decisions at scale.
This article is not advocacy. The AI features gap between Microsoft Copilot and anything in the open-source stack is real. VBA macro compatibility in LibreOffice is partial. Migration has genuine engineering costs that routinely get underestimated. What follows is an honest capability evaluation: what replaces what, where the gaps are, what migration actually involves, and what it costs.
For European cloud sovereignty context, see our European cloud sovereignty pillar. For the infrastructure options that host this stack, see our coverage of EU-native cloud providers that host and support this open-source stack.
What is the open-source sovereign stack, and what does it actually cover?
The open-source sovereign stack is not a single product. It is a bundle of components assembled for interoperability, covering every functional area that Microsoft 365 addresses:
- Nextcloud — files, calendar, contacts, video/chat (replaces OneDrive, SharePoint, Teams)
- Collabora Online / Nextcloud Office — browser-based document editing (replaces Office Online)
- LibreOffice — desktop office suite (replaces Microsoft Office desktop)
- Open Xchange (OX App Suite) — email and calendar server (replaces Exchange Online)
- Thunderbird — email client (replaces Outlook)
- Univention Nubus — identity management and SSO (replaces Microsoft Entra ID)
OpenDesk, assembled by ZenDiS (the German Centre for Digital Sovereignty), is the pre-integrated government-grade bundle of these components. Rather than assembling Nextcloud, Collabora, and Open Xchange independently, OpenDesk gives you a vetted, tested configuration backed by the German government. It has been adopted by Schleswig-Holstein, the International Criminal Court, and others.
There is a distinction that gets glossed over in almost every sovereign cloud conversation. Hosting Nextcloud on AWS Frankfurt or Azure Germany North still exposes your data to US CLOUD Act jurisdiction. The CLOUD Act — passed in March 2018 — establishes that US law enforcement can compel US-incorporated companies to produce data regardless of where it is physically stored. Jurisdiction follows corporate ownership, not data location.
Austria’s Federal Ministry for Economy, Energy and Tourism (BMWET) learned this the hard way. Their Data Protection Impact Assessment under GDPR Article 35 found that CLOUD Act exposure on Microsoft 365 was an unacceptable compliance risk. The Nextcloud migration was the compliance response, not a cost exercise. CIO Martin Ollrom put it plainly: “It’s not only technically possible to deactivate our services, because of the political situation it’s becoming more and more likely.”
For genuine sovereignty, Nextcloud must be hosted on infrastructure with no US-incorporated parent company in the ownership chain. EU-native providers without US parents include OVHcloud, Hetzner, Stackit, Ionos, and MassiveGRID.
How does document editing compare — LibreOffice and Collabora vs. Microsoft Office in practice?
For most routine business documents — writing, formatting, presentations, spreadsheets without complex macros — LibreOffice and Collabora Online handle typical business use with acceptable fidelity. ODF (Open Document Format) is the native format and the safest choice for internal workflows where you control both ends of the exchange.
LibreOffice is the desktop office suite; Collabora Online (marketed within Nextcloud as “Nextcloud Office”) is the browser-based version built on LibreOffice’s codebase. Same underlying engine, different delivery.
The VBA and macro gap. LibreOffice has its own macro language — LibreOffice Basic — that handles simple macros. Microsoft VBA macros do not run natively and must be manually rewritten. For organisations with macro-heavy workflows, this is a hard blocker. Most common with complex Excel VBA automation: financial models, data processing pipelines, COM automation.
DOCX/XLSX format fidelity on roundtripping is imperfect. Complex formatting — advanced table styles, embedded objects, SmartArt, precise headers and footers — can shift. For contracts, regulated reports, or design-heavy documents exchanged externally, you need to test in the specific context before committing.
OnlyOffice is worth noting as an alternative browser-based editor. It natively uses Office Open XML formats (DOCX, XLSX, PPTX), offering stronger document fidelity than Collabora for organisations that regularly exchange formatting-sensitive documents externally. It can be deployed as the Nextcloud editor instead of Collabora.
Schleswig-Holstein’s operational experience is the most useful data point here. The state’s digital minister acknowledged the migration “wasn’t without cost. The process had required significant expenditure” — but document editing for routine administrative work has proven viable at 30,000-seat scale. For a detailed account of France’s Visio deployment and the Schleswig-Holstein migration, including what the transitions actually involved at an organisational level, see our case study coverage.
Before committing: audit your macro inventory. Categorise macros as (a) simple enough to rewrite in LibreOffice Basic, (b) replaceable with a modern workflow tool like n8n or Python, or (c) genuinely blocking. Only category (c) is a hard migration blocker. Most organisations find category (c) is smaller than they expect.
What does real-time collaboration look like, and can Visio actually replace Teams and Zoom?
Real-time co-editing in Collabora Online supports multiple users editing the same document simultaneously with presence indicators and tracked changes. The honest assessment: latency and conflict handling lag behind Google Docs or Microsoft 365 Online for large documents with many simultaneous editors. Collabora Online is also CPU-intensive under concurrent load — infrastructure sizing matters. For typical team document collaboration it works. The limitation shows at the edge cases.
Nextcloud Talk provides built-in video conferencing and team chat. For internal meetings and small group calls it functions adequately as a Teams replacement. But there are limitations: participant capacity without the High-Performance Backend add-on, less mature recording and transcription, and an external guest experience that is functional but less polished than Teams or Zoom. For high-reliability external client calls at scale, supplementing with European-hosted Jitsi or Element is worth considering.
France’s Visio is the flagship proof-of-concept for sovereign video conferencing. Developed by DINUM (France’s interministerial digital agency) and hosted on Outscale — a Dassault Systèmes subsidiary operating under SecNumCloud certification with ANSSI oversight — Visio includes AI-powered meeting transcription and captions processed entirely within French sovereign infrastructure.
The mandate: all 5.4 million French civil servants migrate to Visio by 2027, replacing Teams, Zoom, Webex, and Google Meet. CNRS, France’s national research centre, is replacing 34,000 Zoom seats with Visio by March 2026, covering 120,000 associated researchers. Estimated savings: approximately €1 million per year for every 10,000 civil servants switched from commercial alternatives.
One thing worth clarifying: Visio is a government infrastructure project, not a commercial SaaS product. Private sector companies cannot subscribe to it. What it demonstrates is that a state can build and operate this infrastructure at scale — not that private companies can easily replicate the model. The realistic sovereign video conferencing options for private organisations are Nextcloud Talk for internal use and European-hosted Jitsi or Element deployments.
What do Nextcloud and Open Xchange actually replace for email and file storage?
Nextcloud Files provides desktop sync, mobile apps, browser access, external file sharing, versioning, and granular permission controls at a level comparable to OneDrive for typical business file storage. What it does not replicate: SharePoint‘s intranet and document-management features, Power Automate integrations, and deep Office 365 workflow automation. If your organisation uses SharePoint primarily as a file repository, Nextcloud Files is a direct replacement. If you have built SharePoint as a workflow platform, the gap is more significant.
Open Xchange + Thunderbird replace Exchange + Outlook for server-side and client-side email respectively. For internal email workflows the replacement is largely direct. The gaps appear at the boundary with external organisations: Exchange calendar federation (free/busy sharing with external Microsoft 365 organisations), Teams meeting join links from non-Teams clients, and Outlook-specific features like shared mailboxes with delegate access and message recall all require workarounds.
Nextcloud Groupware (calendar, contacts, tasks) replaces Exchange Online’s CalDAV/CardDAV functionality via open standards. Migration from Exchange using CalDAV/CardDAV export and import is well-documented, but it requires hands-on engineering work and is not a self-service task.
Schleswig-Holstein’s email migration — 40,000+ accounts and 100 million emails and calendar entries — is complete and operational. It was the most complex phase of the full OpenDesk rollout, precisely because of external partner interoperability requirements.
The capability comparison: where does the open-source stack genuinely fall short?
Here is how the three stacks compare across the workloads that matter most for a typical European tech organisation.
Document Editing: Microsoft 365 has native DOCX/XLSX, full VBA, and deep format fidelity. Google Workspace handles DOCX import reasonably well but has no VBA. The OpenDesk stack — Collabora or OnlyOffice — is adequate for routine documents but carries the macro/VBA gap and imperfect DOCX roundtripping on complex formatting.
Real-Time Collaboration: Microsoft 365 Online is mature and handles large concurrent documents well. Google Docs is the benchmark for concurrent editing. The Nextcloud/Collabora stack is functional for typical team collaboration but lags at scale with many simultaneous editors.
Video Conferencing: Teams is mature, with recording, breakout rooms, and external federation. Google Meet is reliable, though it lags Teams on enterprise features. Nextcloud Talk covers internal use adequately but has participant limits without the High-Performance Backend add-on and less mature recording.
Email: Exchange has the full enterprise feature set — calendar federation, delegate access, all of it. Gmail has strong calendar sharing across organisations. Open Xchange and Thunderbird cover internal email solidly; external calendar federation with Microsoft 365 organisations is where the gaps show up.
File Storage / Sharing: OneDrive and SharePoint handle intranet, Power Automate, and deep Office integration. Google Drive is simple and well-designed. Nextcloud Files is a strong OneDrive replacement; SharePoint’s intranet features are not replicated.
Identity / SSO: Microsoft Entra ID has broad SaaS ecosystem integration. Google Identity is widely supported by SaaS vendors. Univention Nubus provides LDAP/SAML support but third-party SaaS integration requires per-application configuration.
AI Features: Microsoft Copilot is integrated across Word, Excel, Teams, Outlook, and SharePoint. Google Gemini is integrated across Docs, Sheets, Meet, and Gmail. Nextcloud AI Assistant provides file summarisation and basic text generation; it is not integrated across the stack the way Copilot is.
The AI features gap is the largest gap in this evaluation. If AI productivity features are a key business driver for your organisation, weight this heavily. Copilot offers document summarisation, email drafting, spreadsheet analysis, and meeting notes within the apps you already use. Nextcloud AI Assistant is not in the same class. This gap will not close quickly.
Identity/SSO is a meaningful practical challenge. Univention Nubus is a viable Entra replacement for internal SSO across Nextcloud and Open Xchange. The practical challenge is third-party SaaS integration: Salesforce, HubSpot, Jira, Slack, and similar tools expect Microsoft or Google as the identity provider. Nubus integration requires per-application configuration for each. For an organisation with an extensive SaaS stack, this is a real upfront integration cost that must be factored into migration engineering estimates.
What does migration actually involve, and what breaks first?
The most common mistake is trying to cut over everything simultaneously. A phased approach avoids the most common failure modes.
The adversarial interoperability framing — articulated by Cory Doctorow and the EFF — is the most useful lens for planning. Genuine migration does not just require open-source replacement software; it requires tools that work without cooperation from the incumbent. Data export utilities, format converters, and identity migration tooling must be treated as engineering problems to be solved, not assumptions. Applied to Microsoft 365 → Nextcloud migration, the process relies on tools — PST/MBOX export, batch format converters, CalDAV migration scripts — that must function regardless of whether Microsoft makes the process convenient.
The five primary migration blockers, ranked by severity:
- Macro/VBA inventory and rewriting — audit your macro inventory before cutover; this is the most common genuine blocker
- Exchange calendar federation with external partners — free/busy sharing with external Microsoft 365 organisations breaks
- Third-party SaaS identity integration — per-application Nubus configuration for every business-critical SaaS tool
- Document format fidelity for externally-shared documents — regulated reports and contracts require testing before cutover
- User adoption and retraining — 63% of organisations cite user resistance as the primary barrier; parallel running during transition reduces disruption
Migration sequencing. Start with file storage — migrating shared drives from OneDrive to Nextcloud Files carries the least disruption. Document editing comes next, with parallel access to both environments. Video conferencing and team chat follow. Email and groupware are last, because this phase carries the highest technical risk: external calendar federation breaks, and coordinating the cutover with external partners requires active communication.
Total cost of ownership — a 200-user comparison:
Licensing over three years runs roughly $10K–$15K for Nextcloud Enterprise plus Collabora or OnlyOffice, against approximately $158K for Microsoft 365 Business Premium. EU infrastructure hosting adds around $24K–$48K over three years for the Nextcloud stack. Backup and compliance tooling adds roughly $6K for the open-source stack versus $15K–$30K in add-ons for Microsoft 365. Migration engineering — data export, macro audit, identity integration, training — is a one-time cost in the range of $15K–$40K depending on complexity. The three-year total lands at roughly $55K–$109K for the sovereign Nextcloud stack versus $173K–$188K for Microsoft 365.
The MassiveGRID TCO analysis provides the lower bound of that Nextcloud range: $68,684 over three years for a 200-user organisation, versus $188,580 for Microsoft 365 — with break-even at approximately month 6. That model uses a $5,000 migration cost baseline. For an organisation with significant macro inventory and extensive SaaS integrations, $15K–$40K is the realistic figure. Underestimating migration labour is the most common mistake.
As Dirk Schrödter put it: “We’re in the habit of exporting public money out of Europe, and importing dependencies.” The migration is an infrastructure policy choice as much as a product decision.
FAQ
What exactly does OpenDesk include, and how does it differ from just installing Nextcloud?
OpenDesk is the ZenDiS-assembled bundle — Nextcloud, Collabora Online, Open Xchange, Thunderbird, and Univention Nubus — pre-integrated and tested for government-grade deployment. Installing Nextcloud alone gives you files, calendar, contacts, and talk. Collabora or OnlyOffice and Open Xchange must be added separately, with integration handled independently. OpenDesk trades component flexibility for a vetted, supported configuration.
Is LibreOffice actually good enough for a real company in 2026?
For most business document workflows — writing, presentations, simple spreadsheets — yes. The honest caveat: organisations with complex Excel macro workflows, or that regularly exchange formatting-sensitive documents externally, should run a structured pilot before committing. Schleswig-Holstein’s 30,000-seat deployment for routine administrative documents is the best available evidence that it works at scale.
What is France’s Visio platform, and can private companies use it?
Visio is developed by DINUM, hosted on SecNumCloud-certified Outscale (Dassault Systèmes subsidiary), with ANSSI oversight and French AI-powered transcription. Mandated for all 5.4 million French civil servants by 2027; CNRS is replacing 34,000 Zoom seats with Visio by March 2026. Visio is a government infrastructure project — private sector companies cannot subscribe to it. It demonstrates that sovereign video conferencing at scale is technically feasible, not that it is commercially available.
What is the AI features gap between Microsoft Copilot and the open-source stack?
Microsoft Copilot integrates AI across Word, Excel, Teams, Outlook, and SharePoint: document summarisation, email drafting, spreadsheet analysis, and meeting notes. Nextcloud AI Assistant provides file summarisation and basic text generation — not integrated across the stack the way Copilot is. This is the largest capability gap in 2026. If AI productivity features are a key business driver, weight this heavily in your evaluation.
How much does migrating from Microsoft 365 to Nextcloud actually cost?
The three-year total for a 200-user organisation: approximately $55K–$109K for the Nextcloud sovereign stack versus $173K–$188K for Microsoft 365 Business Premium. The MassiveGRID analysis puts break-even at approximately month 6. Migration engineering — macro audit, identity integration, data export tooling, training — ranges from $15K–$40K depending on complexity. The cost case is favourable; underestimating migration labour is the most common mistake.
What is VBA macro compatibility like in LibreOffice?
LibreOffice Basic handles simple macros; Microsoft VBA macros must be manually rewritten. For organisations with complex Excel automation, this is the most common migration blocker. Audit your macro inventory before committing to the migration.
Is running Nextcloud on AWS or Azure still sovereign?
No. The CLOUD Act means US law enforcement can compel US-incorporated companies to produce data regardless of physical location. AWS Frankfurt and Azure Germany North do not resolve this. For genuine sovereignty, Nextcloud must be hosted on EU-jurisdiction infrastructure with no US-incorporated entity in the ownership chain. The Austrian BMWET DPIA reached exactly this conclusion — and migrated accordingly.
What identity management solution replaces Microsoft Entra in the sovereign stack?
Univention Nubus provides LDAP directory services, SAML 2.0 identity federation, and user lifecycle management. For internal SSO across Nextcloud, Open Xchange, and intranet tools it is a viable Entra replacement. The practical challenge is third-party SaaS integration — Salesforce, Jira, Slack expect Microsoft or Google as the identity provider, and Nubus requires per-application configuration for each.
For the infrastructure layer that hosts this stack, see our coverage of EU-native cloud providers that host and support this open-source stack. For broader strategic context, see our European sovereign cloud strategy pillar.
