The wrong way to use Slack is as a substitute for email.
This is when you create separate channels based around departments (usually with one general group for everybody) and tell everyone to participate there instead.
The benefit is now everyone can see, search for, and participate in discussions.
The downside is this becomes an overwhelming amount of information, it drastically increases the time spent communicating (this is not a good thing), and everyone is invited to share opinions regardless of their experience or expertise. Your decisions change to accommodate the opinions of people who shouldn’t be influencing the outcome.
A better way to use Slack is as part of an advanced collaboration process that cuts communication, not as a substitute for email that increases it.
When this system works well, it lets you coordinate tasks, track progress, reduce time communicating, and ensure your team are working to their strengths.
Remember the goal isn’t to increase communication, it’s to reduce it.