The desktop app is crap.
I dare you to scroll up in a chat with links, images, or gifs. It's so yanky, it's impossible to find anything.
Or search for a message you know is in a specific context. You will not be able to open the message in its context, only the message itself. Useless.
Or call somebody and then try to use the media keys on your keyboard. They won't work and instead play the Teams ringtone twice (or sometimes infinitely).
Or check the online status of your contacts. Spoiler: they are accurate only 50% of the time, sometimes showing a different status of a person on the same screen (e.g. contact list and chat).
Or try to switch chats quickly. It is so unnervingly slow.
Or try to react to a new message with an emoji with 125% scaling on. You cannot do it because the selection popup is like 30 pixels high because it opens downwards, not upwards.
Or try to share a 2560x1440 or higher screen with more than 0.5 fps over a gigabit connection.
Or try to remotely control another PC over Teams screen share and press Ctrl+, or Ctrl+Shift+Space or simply Tab. All those are getting caught by Teams, sometimes they are forwarded, sometimes not, but they always call a function in Teams and open various menus. All those are integral shortcuts in Microsoft Visual Studio, by the way.
Or try to copy text of a message you got, you will always have the name of the sender and timestamp in the clipboard, which requires me to paste it into a text editor first to copy and paste only the text into e.g. a single line search bar.
Or try to compile and just use Teams at the same time. MSN was more performant on a 450 MHz PC 20 years ago.