Microsoft Teams has become so popular it may actually be illegal.
I've never used Slack, so I don't really know how it compares to Teams, both in UI/UX and feature-set, but it must be better than Teams, which I am familiar with. The Teams UI/UX isn't the worst I've ever seen, but it's not very good either. For example, the "back" button doesn't just go one step back, it goes "all the way" back, often making much more work to get where I need to be.
Also, if someone is trying to call me/invite me to a meeting, it starts playing an audio alert, but I have yet (after over a year of using it) to find any sort of visible notification of who's calling or how to join the call. There's just a phantom "ringing" sound playing on a loop, but I seemingly have no way of knowing who it is, and no easy way to answer. So if I'm not already expecting it and know who's calling me, I basically just have to ignore it. Now maybe I'm doing something wrong, but I feel like an incoming call should be sort of in-your-face, and not require forensic analysis to figure out what's going on.
There's a bunch of frustrations that have led me to avoid using it as much as possible, rather than get more familiar with it.
(Disclaimer: I am not a financial advisor / this is not financial advice)