Teams because already use office / exchange and teams is integrated and “free”
We primarily use slack for communication, so I don’t have to use teams much, except for meetings
It’s all just tools and they work reasonably well if you use them as intended.
I don’t share your views on policies though, it’s important that people don’t do their own security assessments and follow what the ciso / security architect has intended. If you disagree, take it up with them.