In practice self hosted usually translates to more downtime and slower performance when it works. Unless your org has more expertise running a chat service than Microsoft or slack, your self hosted alternative is always going to suck more.
Did you try self-hosting and it lead to more downtime and slower performance?
From my experience, when I self-host stuff it's a lot faster (more server resources) and never had any downtime (server doesn't simply go down for no reason).
I haven't tried it personally. But my employer hosts an on-prem Github instance and it is just terrible. So many downtimes, long times before anybody gets around to repair it, general performance issues, maintenance windows for upgrades, etc. Just a huge pain. I've seen this sort of problem with the old Exchange on-prem services too.
