Why is Discord always ignored in discussions of slack alternatives? It's literally a slack clone. Are you all just put off by the marketing or the fact that it's not exclusively used by 'professionals'?
You can't enforce password length requirements, or whatever your security team recommends this year.
You can't disable parts of the offering, like voice chat, to enforce people use hangouts or whatever your company wide solution is.
At the same time, things companies may want employees to do easily but gaming communities don't want, such as allowing any user to create a channel are not possible with Discord's permission model.
If you want users to see only self-selected subsets of information, you can't really. You could break your company's chat across multiple servers, but this is tedious compared to just having channels users can join or not. For example, in my last employer, I was in 40 or so channels out of several thousand.
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I think for a startup or small business, none of these things are that important. But for medium or large companies, they become more of a problem.
I use it at home and it does occasionally go a bit weird and need a reboot, or stop sending notifications on my phone. Not a big deal for me.
Slack should definitely add a 'forum-like' kind of channel before someone creates a new 'forums with native client' product.
I personally don't use it because everywhere I've seen it previously is in gaming/twitch communities.
2. Customer service doesn't respond quickly even with nitro plan
3. Confusing UX