That Teams requires you to put every channel in a team is a huge pain. We often have orthogonal needs (teams vs projects) and need cross-team adhoc channels. When I was with an organization that used Slack, this was easy. With Teams, you have to figure out where to put a channel and who is on that team. You also cannot tell who is in a channel because you join teams, not channels. I miss Slack's ability to spin up a cross-team project channel and just invite whoever needs to be involved.

Also, whenever you create a team in Teams, it creates a SharePoint site for that team. So we are the engineering team and want all our docs in engineering. But to spin up a cross-team project team means it gets its own SharePoint site and now files are scattered. Want to add a Loop workspace? That's per channel, not per team. And teams are exchange groups - so it makes handling exclusive email groups more difficult because if your team is public then anybody can join your email group.

That's my biggest gripe about Teams. But also notifications have never worked well for me. The integrations, even with Microsoft products, are poor. Want to send a well-formatted Azure Monitor alert to a Teams channel? You have to set up a complicated and fragile logic app (power automate) and figure out how to transform the message from the "common alert schema".

And message management is harder. In Slack I could always use the built-in remind-me-later. It'd put the message in Later and notify me again. The best we have in Teams is the power automate workflow to resend the message. But it's just too much friction typing in the exact date and time I want it resent vs Slack where I could just click "remind me tomorrow".

End rant