Comparison

Mattermost.com vs Slack

Mattermost.com and Slack both land on Depends.

Mattermost.com

Depends

Confidence: Medium

Buy if you're a regulated, defense, or government team that must self-host chat and has ops staff to run it.

Depends

Confidence: Low

Confidence is low: only Slack's own marketing/security page was available for review.

Mattermost.com versus Slack
Compare	Mattermost.com	Slack
Verdict	Depends	Depends
Best for	Defense and government contractors	Distributed teams coordinating in channels
Who it's not for	Small teams that just want a group chat	Solo founders or 1-3 person teams . platform weight is overkill
Privacy	Active security program with bug bounty and routine scans, but several recent CVEs are disclosed, including an auth bypass.^⁸	No known public vulnerabilities found in the sources reviewed.¹¹
Support quality	No support quality evidence found	No support evidence in sources reviewed
Public sentiment	Engineers praise the self-hosted threading model while users report slow mobile apps, message-history paywalls, and shrinking free features.^⁴	No independent user reviews were found in the sources reviewed.
Biggest gotcha	Entry edition locks message history after 10,000 messages . older messages sit behind paid plans.¹²	Guess: Enterprise Key Management is likely a paid add-on . confirm before budgeting.¹¹