My experience with Slack was it is a litmus test for your company culture. If you have people prone to histrionics and complaining, it's going to encourage that. If you have cliquish attrition-game players, they're going to create their own parallel channel scheme. If you have grandstanding drama queens who work an audience, it's the weapon of choice for the sort of people who send emails with long cc' lists. If you…
To that end, Slack, being a communications enabler, merely accelerates the rate of these things happening. You can turn it off, but then you can turn off the company too. You can try to make everything out in the open open, but then you quench creativity as well as meaningful 1-1 interactions between people.