I missed messages and conversations all the time when I had to use it
Bingo, that right there. Need to work with one team, but also quickly jump onto a meeting with another client... So bad. If you need to be available to two clients at the same time? No, that's not going to work.
The UI is horrible, finding people is difficult at best. Chatting with someone and want to jump on a video call... well, that will take a few minutes to locate the right button. At least Google Chat just put a big old button right in your face so you can do a quick meeting. Google suite of product (Meet, Calendar and Chat) have their own issues, but it's so easy to do meetings, and it just works every time. For six months a number of us at the office were unable to make audio work on the desktop Mac app, and Firefox wasn't supported, so you needed Chrome to join a Teams meeting.
You can't really complain about the audio or video quality in Teams, they got that mostly figured out. It's just that the desktop app has a horrible UI. I mean so does Slack, given the option I'd use neither.
I don't use the local Teams client.
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.