I had 2 clients each using Teams, and I was working with each. But... I can't just be 'logged in' to two different Teams organizations. If I was in company A, I couldn't see anything from company B. Apparently you can do this in the phone app, but not the desktop version. I could keep one company on desktop, and another on phone, and just monitor two, but... you can't scale that.
With slack, I'm just connected to 3-4-5 client orgs at a time, and can react/respond as needed within the same tool without needing to log in/out constantly.
The last time I had to deal with this was last autumn - perhaps it's "fixed" now? Except... I don't think it's seen as a "bug" in the first place, so may never be "fixed".
Also... just connecting to Teams would often just hang... wait... no indication anything is happening. If I got impatient, I'd have folks saying "oh... just calm down..." but then also wonder why I was "not in the meeting"... well... because... Teams can never tell me if it's going to take 90 seconds to join, or 39 seconds or... if it just will never resolve. Opening a secondary link in a web browser became my default, though I'd have to read a banner every time saying "you won't get the best experience - some features may not work!" except... it at least loaded.