Teams is free as part of an Office 365 subscription (just like Sharepoint and all the other crap they bundle with it), which is what definitely makes it an existential threat to Slack, because now Slack is competing with Microsoft's very strong Office monopoly.
The fact that you can create a group with some channels, add your files there and edit them together, have a wiki, have a (not very good) kanban board etc. makes it a pretty complete experience. Some of the tools they release needs more polish and some tools are by themselves much worse than stand alone competitors but having everything in one is pretty nice.
Then there is sharepoint, oh, and onedrive, and they all come in the same package so everyone uses something else.
At a previous company, we had slack and MS Teams side by side, to trial out MS Teams. Everyone preferred Slack by a huge margin. We were told to get use to Teams, because Teams is free.
Guess what, it was fine.. no notable productivity lose going from slack to teams. If anything, team's sharepoint file integration is much better than slack, for keeping a single source of truth.
The Android app in particular has trouble. And muting is all-or-nothing.
I've also had corrupted channels, I even escalated to Microsoft once.
Doesn't matter, since it comes with Office so it basically costs $0.
It's crap, but that's not stopping my employer from replacing literally everything they can with it. Even our desk phones, purportedly.