I think what turned me off was the headline. If it had been more well-scoped, e.g. "Slack sucks balls when it comes to doing real work", I'd be less complainy. I've enjoyed Slack for conversation in medium-ish interest groups/clubs (100 <= x < 500), in which the channels aren't overwhelming and there's almost never a need to do private chat with more than one other person.

Everything in the context of work loses its allure, such as "unlimited" vacation time.