I don't really understand where this notion comes from. I've used Slack; I've used Teams; I find Teams more than adequate for my organization's use-case. I'm sure that's not the case for everyone, but "Teams is complete shit" is such an odd over-generalization from my vantage point.
For me it is the slowness and bugs. I complained on occasion about Slack's responsiveness but Teams on my Mac is on another level of unresponsiveness. Also it doesn't seem to use the macOS APIs for notifications, Teams notifications show up even in Do Not Disturb mode. Tapping the notification is hit-or-miss as to whether or not it will actually load the relevant discussion.
Teams is also really bad at emoji support compared to Slack.
I've been using Teams for only a few months after having used Slack for years but in the time I've used it I've come to despise it.
