I used Teams just before Covid (last job I had used it). For one messages didn't arrive in the same order for everyone, so chat histories often didn't make sense, with replies appearing above the messages they were replying to. The other thing I recall was slow loading time. Slack is _snappy_, moving from channel to channel is very fast, as if nothing ever needs to load (it does, it just cleverly preloads everything except images it seems like). The interface was just far less intuitive as well.

At the end of the day, just about every team that I worked with had a WhatsApp group that they actually used to chat in. Having a bad product as your internal chat is how you get shadow IT like WhatsApp where people are discussing your proprietary information on a third party service.