I like that Mattermost treads Channels as Groupchats, and you can reply to a Chatmessage or just add to the Conversation where in Teams you have Conversations where you can reply to or start a new one, this leads to people replying via new conversation or replying while it's a new conversation.
I also dislike the notion of Teams with Subchannels in Teams and the hard cut between Chat and Teams - I like the way Mattermost does it more: a <hr> Element between Channels and Chats.
I think it's widely hated because its absolute hot garbage from a UI/UX. Slack is bad enough from a UX perspective (not necessarily Slack itself, since the UI is nice, but from an overall experience of "interrupt anyone anytime"), but Teams took the concept of direct messaging and instantaneous communication and then arguably made it more frustrating.
I also dislike the notion of Teams with Subchannels in Teams and the hard cut between Chat and Teams - I like the way Mattermost does it more: a <hr> Element between Channels and Chats.