I don't mean to just blanket shit on Teams, but Teams is just a confusing mess of UI choices and UX design that makes no sense even within the context of using Teams. The meeting icons are of course pretty awful as you cited, but it's even more things for me [0]:

- When joining a Teams call, the toggle for video gets "selected" so that pressing Return or spacebar (I think one or both) will toggle the video on -- noticing that you did this or that the video toggle is selected is a matter of chance as it's hard to see

- For some bizarre reason Teams has a "start call" shortcut that just immediately starts a call without the usual pre-call warning items. Joining a meeting from your calendar gives you a "pre-meeting room" where you can confirm your mic/video settings before joining, but hitting the call shortcut or button immediately starts a call

- Sometimes right-click menu loads slowly and additional options load after you right-click and move the mouse -- it so happens this will usually put the cursor on Pinning the message instead of selecting reply or edit

- Regarding Reply/Edit, there is a nice button to jump right to both, but for chats one button is showed, for private messages another is shown

- All teams messages are linkable; whether or not you right-clicked on a link in the message and are copying the link or if you're getting a link to the message itself depends on if you happen to notice whether you have 2 options on right-click or 3+ options

- Copying a linked item (e.g., document, media, picture) will have Download or Copy Link button. Copy link for some reason puts up a text box across the conversation you're having that is dismissible with escape or clicking usual x in box corner -- other "copy link" options just copy the link normally, other ones (like copying channel link) will open a window with the link for you to copy

- it is huge pain for me personally that the links you copy from Teams are Sharepoint links and pasting it in a browser tries to open files in Sharepoint browser, even if Sharepoint absolutely cannot display a preview of the file: you sit while Sharepoint tries to load a preview, and only after a few seconds of Sharepoint trying does it show you a download button to get the file (thankfully there are browser extensions like Redirector [1] which can be used to create redirects for auto-downloads...just Microsoft likes to change the URL for actual downloads relatively often so occasionally you need to update your redirects..)

Teams is so inconsistent and the UI and UX are equally inconsistent -- Teams is also not shy about showing tutorial prompts for features just whenever it wants to, no matter how long you've been using Teams, sometimes it will just block the entire app to highlight some feature it wants to advertise. I honestly don't think this or anything has to do with flat UI versus other ones, it's just plain lack of attention. maybe flat ui's give the impression of a "completed" thing, but I just can't see that most of the UI/UX issues for apps like Teams are about the aesthetic so much as just a complete lack of concern over what actually using the app is like.

0 - All points here were observed on vanilla teams installations on different computers -- maybe my work just has weird defaults, but I'm not confident that is the case

1 - https://github.com/einaregilsson/Redirector