Teams isn't just mediocre, it's aggressively hostile to basic usability. The camera bar sits at the top of the window, directly blocking where you're supposed to position your camera for eye contact. Chat organisation is broken: you get duplicate groups because the order people were added matters somehow. Notifications phantom in and out. Reactions are buried in an activity feed. Search is useless. You can't reliably paste text without major formatting issues. The mobile app logs you out randomly and doesn’t tell you unless you manually check it. Desktop notifications don't sync with read state. Files uploaded to chat don't appear in the Files tab. The "new Teams" broke half the features that worked in classic Teams. Presence status is a coin flip. Audio settings reset themselves between calls. Screen sharing has a 50/50 chance of sharing the wrong window. The difference between a chat and a channel is arbitrary and confusing. You can't edit messages older than a few hours. Threading is bolted on and barely works.
Performance is inexcusable. Multiple gigabytes of RAM to display text messages and lag constantly on modern hardware. How do you make a chat application lag? It's rendering text, not computing fluid dynamics. Opening the application takes 30 seconds on an SSD. Switching between chats stutters. Typing has input delay.
The real problem isn't that Teams is terrible. It's that "it technically functions" has become an acceptable standard. When you've never experienced better, "it works" seems fine. But Teams is what happens when a monopoly position means you don't have to care about quality. Microsoft has unlimited resources and still ships this.
Even Skype for Business was more stable, and in Skype for Business you couldn't reliably select text. That's how low the bar is.