Teams is worse in that it appears to actually be Electron instead of the semi-native (I don't think it's pure Win32, judging by the UI latency and appearance) Skype for Business.

Being awful is Skype's killer feature. You can't carpet-bomb with pings, you can't comfortably copy and paste code or logs into it, half of devs who use linux don't bother running it on a VM and instead use flaky third-party clients, it eats messages randomly, so long story short - people generally avoid it except for trivial shit. And on the odd day when they don't, I get a copy on Outlook anyway.

I've read some horror stories about Slack, and I shudder to think what would happen if my company moved to it.

only for the cloud version.

S4B server edition is still in active dev