I've used a lot of chat apps over decades. Teams might be the first one that manages to be _confusing_ - a feat I'd never seen in a chat app - because of the way it rearranges new threads, yet hides the recent part of the thread. Plus various other half-implemented features. The wiki and sharepoint/file integration doesn't work very well. There's no wiki search. It's super easy to delete an entire wiki. I've lost notifications on entire channels for > 1 month without realizing it. Just a ton of little stuff like that; my team will never go back.
Slack is "fine" - it mostly works, and has a lot of app integrations.