My Fortune 500 healthcare company successfully uses Mattermost because the lawyers aren't comfortable with us using slack. It's been great. Thank you @it33 and team.
p.s. we haven't run into the issue 6320 (users archiving channels), for exactly the reasons @it33 described in the issue report. He didn't explain it well, but permissions is a complex rathole, un-archive is a simple workflow for admins and if there's continued abuse, just ban the user.