Lots of weird polish issues in my opinion... One that really peeved me was (for a while at least) you could search for a message, but there was no way to jump to that message from the search results. So you couldn't read the context unless you scroll all the way back up.

But primarily it's that the mechanics are different from things like Slack and Discord in ways that are just less intuitive.

Channels function more like announcements + comments rather than a chat—you are really shoehorned into posting a "Topic" and discussing it in the replies. There's no way to carry a linear conversation in a channel otherwise. And to load replies you have to keep clicking "see more" as if this is a social media site, so it's very annoying when your 800+ comment critical discussion happens there. Not to mention notification settings aren't granular enough, so you either get hammered by all activity, or remain oblivious to discussions which may have popped up in an older Post.

What tends to happen in my experience is small working groups spawn off a group chats because the flow is better for daily conversation there than in Channels. Which, of course hides this activity from anyone not in the chat. And group chat's are entirely linear in Teams—you don't have threads the way you do in Slack, so chat history tends to get messy quick.

The channel-then-thread organization Slack uses is much more natural for the teams I tend to work on, because you just have the one main discussion which can be segmented into threads as needed.