Wide ranging differences in opinion...
At work have to use Slack, Teams, and also Zoom on occasion.
Teams seems dismal for chat and keeping up with what's happening in wide range of differing conversations. Would hate to have any sorts of alert notifications coming through Teams.
Slack has been working much better overall, particularly in a distributed WFH setup as we are currently in. Mind you, it could be related to the sorts of conversations that happen in each option. Working in Teams during the outage has been painful, but again, might be because we are trying some of the conversations that would normally happen under Slack.
One of the other painful bits around Teams is that it is tied in to the Office365 authentication, which gets painful when you have multiple clients with different domains that want you to authenticate to different Office365 accounts. I might be using it wrong, but it doesn't seem obvious to manage the differing authentications as easily as Slack workspaces.
Still, someone pays some money and you get less choice...