But you're right, the best competitive advantage of Teams is that it's "free"[0]. But that's about it, it's objectively bad in all other ways - anyone claiming otherwise has Stockholm syndrome and is in denial =)
Just the fact that, as a consultant, it's literally impossible to connect to multiple teams workspaces with the same client. The OFFICIAL Microsoft advice is to use Private Browsing in separate browsers...
Meanwhile on Slack I can just hit cmd-number and swap between clients. AND we can have shared channels over instances, like $vendor having a support channel on our Slack with their actual people with their own accounts.
[0] If you have M365 anyway.
Thankfully we use Slack where I work. The cross-org channels are cool.