I can’t really comment on video calls in Matrix since I never used them in Slack either. For me the main draw is having one tool that does one job well, rather than trying to be the all-in-one hub for everything. I’d rather have messaging in one place, email wherever it lives, and video calls on a separate tool that’s actually good at that, instead of relying on a centralized system that tries to cover all bases but ends up being mediocre at most of them.