I've been using Teams at college for over a year now and at work for a couple months. It baffles me that there are simple, obvious bugs that have remained all that time.
For instance, on the Windows desktop app, the word "I've" gets marked as incorrectly spelled. When you click on it to see the spelling suggestions, it suggests "I've". Clicking on the suggestion does nothing and it continues to flag it. This has been an issue in the app for over a year and I refuse to believe that the developers are unaware of it. It's a very common word to be typing.
Another problem is sending files or images. You have to wait for it to finish uploading before it will let you send the message. Not only is it pretty slow (I would estimate 1MB/s, whereas Discord uploads at my full 12MB/s), but sometimes it won't let you send for a couple seconds even after it finishes uploading.
A couple months ago Teams added read receipts, which is really nice, but they don't always work. My work has them globally enabled and everyone is on the latest client, yet each person only sees them for certain other people. I don't see them for anyone, but my coworker sees them for about 50% of our staff.
Notifications are also buggy. Teams will just randomly decide to not give you notifications for messages or calls. I've missed multiple messages in Teams for days because it never alerted me. I had to actually open the specific chat with that person before I saw the message. I've gotten into the habit of checking Teams every 15 minutes because of this. Teams for Android also seems to send notifications a good 30 seconds before the desktop app does, so I usually keep my phone on my desk solely for Teams notifications.
I would also like to point out that Microsoft built a general-purpose notification system into Windows, yet Teams uses a completely custom notification system. This completely baffles me as they aren't even following their own company's best practices.