We used Teams at the last org I worked at, we rarely had issues. The UI wasn’t bad in my eyes, we had integrated our phone system into teams too. Calling someone on the other side of the country via teams was simple.

Admittedly the service did get worse before I left and there were a couple of Microsoft outages which cut all our text based communications for a day (phone system still worked)

I’m using slack right now but would prefer something self-hosted but that’s well out of the scope of the current mob I’m with.