Slack has built-in calling functionality, but my team always uses Zoom. Zoom feels more reliable, but I'm curious if many teams get by with just the Slack Calls feature.
Slack's video calls are best for short, quick calls with a few people. I've found them most useful when I need to call one other team member, for calls that otherwise would have been phone calls. One huge benefit is that Slack calls don't have their own text chat—you use Slack's normal chat instead if you need to share something—so you don't lose notes in conversations after the call, as you can in Zoom if you're not careful.
But Slack video calls are capped at 15 participants, so if you have a larger team call, you'll still need Zoom. Slack also seems to be a bit more resource intensive and less responsive on slow internet connections than Zoom, though your mileage may vary.
It's hard to beat Zoom's video quality, from our testing, but Slack's calls being built-in do make it a handy alternative.