They're generally fucking terrible in terms of UI. I much prefer Google Groups over older forums. Discourse is even better but there's no easy way to setup a forum for free. The last time I looked into it it would be roughly $10 a month for a DigitalOcean droplet to run a small forum, but that was more than I wanted to spend myself. And accepting money is a non-zero amount of work too.

Probably the main reason why Slack – and Gitter, and IRC too still – are widely used by open source projects is that they're free, financially, and mostly free in terms of maintenance.