It wasn't actually so bad at the start, it was just a poor slack knock-off. (Poor because the visual density is much lower with these oversized chat bubbles)
But now they are spending all their dev time cramming so much stuff into it without considering performance that it's become tooslow and cluttered. Now it's a wiki, a file storage thing, a video chat app, an interface to Yammer, not to mention the tons of plugins. Cool idea to have one app that does everything but performance and usability have gone down the drain. It feels very beta to me.
Oh and the choices they make are so stupid sometimes. When I decline a meeting it still adds me to its associated chat and pings me for everything that's said in it. Constant distraction and no way to turn off this stupid default.
I don't see why I should use a Microsoft GUI any more either. Once you stop using their libraries, you no longer need their OS at all. Pretty strange behaviour from MS if you ask me, encouraging your users to get off their systems.
Microsofts successful cross platform apps like VSCode and Teams ignore all of them so following suit seems pretty sensible to me.