My personal favorite UX-failure-of-the-moment in Teams: If I open the teams tab > Browse, it shows a big list of company-wide channels that I could join. There's a search box, but unlike any normal search box, it only does a prefix search, so if your channel is named "some test channel", and you search for "test", it doesn't find it! Several times I've given up at guessing the right channel name and had to ask coworkers to tell me the exact name in order to join.