After leaving slack entirely from a robotics team and a science club, our productively increased.
No longer were we going off topic wasting precious time and making everyone in the channel read every 500+ messages some people engaged in. We instead chose to make more plans during meetings and through direct chat (Messenger).
However, Slack is usefull for forums such as donkeycar.slack.com where many hobbists come in and have questions on this software/hardware project. Slack is easily accessible for anyone to ask questions and with proper use, I was pretty productive in communicating effectively across channels.
Despite this, I believe that slack in general is a waste of time in a TEAM. Why not have meetings or meet up in real life rather than hiding behind a screen without any physical interaction? Slack, however, is a great alternative for forums since there is great searching capabilities within channels and there's always someone there to help.