My favourite file access "feature" of Teams (I verified it was in fact working as intended, at least until the pandemic and the warts started getting more obvious) was that class teams had read/write access by default for all files in a team. This meant every student in a class team could modify what you uploaded by default. Of course fixing this required opening up the team in Sharepoint and fiddling with permissions, totally something every teacher is expected to figure out right?
Not really a software feature, but their update rollout style is awful as well. Announce features 4-8 weeks in advance of rolling out patches. Inconsistent rollouts, so my home desktop might have the latest feature patch, but my work laptop won't (I was around 4 weeks out of sync at one point). Manual checking for updates won't apply the latest patches. Meanwhile their consultants in education are crowing about all the new features or bug fixes.
I actually like some of the feature set, and it's very useful in an educational setting now they've brought some of the new features online (3 months later than would have been useful for the pandemic lockdown in my part of the world, but oh well), but enough frustrating elements that I'm constantly supporting workmates in its use.
Wait, what? Do threads move when they get replies or something?
If you are trying to get students to do work, maybe use something like planner, sharepoint or any of the other myriad of tools actually designed for collaborative working rather than chat.
A UX that encourages replying out of thread because it's too darn confusing.
Unable to quote messages on desktop, but you can on mobile.
Unable to be signed into more than one Team. Have they never heard of consultants? At one point last year I had 4 separate Teams I needed to be signed into. Microsoft's solution have different Chrome profiles (Yep, Chrome, not Edge) for each one. My laptop only has 16GB of ram, so that didn't last long...
Super unclear UX around document viewing. I open a file in Teams and if I have write access I am instantly saving all changes. So many times I've shared a document for feedback and then had to recover the original version from Sharepoint because people changed a lot of things without Track changes on.
Based on Sharepoint. 'nuff said.