One main reason why MS Teams sucks hard:

"Teams" force every discussion to be a thread.

"Chats" don't have threads

Result is that people only use chats and they are one of:
a) a jumbled mess of topics
b) things getting ignored
c) discussions hidden on 1:1s

And don't get me started on how the MS Teams UI seems designed to enforce silos inside organizations.

In Slack it's easy and you are encouraged to seek out other channels and join them/leave them as needed. In Teams it's assumed that you'll be given access to Teams.

Overall MS Teams seems like a product of the previous generation's idea of how a Workplace should work. It feels old