Could your view be slightly tinged by being required to use it at work?
I've used Slack for a couple years in purely open source / social / educational settings and can't really fathom how it could be described as comically bad. There are aspects I dislike, but overall it feels like it solves pretty well the problem it set out to address.
A lot of the hate seems to be around the the fact that it gives employers yet another method of disturbing work flow and creating accountability around meaningless metrics (response time or participation in pointless chats).
Instead of increasing productivity, it turns the entire workday into a meeting, with all the downsides of a traditional meeting.
But, again, those are all complaints about the setting and expectations surrounding the product, not the product itself. Slack is quite good at what it does, by any reasonable measure in my view.