Ask HN: Best open-source alternative to Slack?

The idea of ceding total control over internal messaging to an external entity (especially for a startup or small company) has always gotten to me, but I’ve recently considered the possibility of using a self-hosted, open-source alternative to Slack. From what I’ve found in research, Rocket.chat seems like a good choice and relatively “easy” to set up. Do you use any self-hosting or open-source options? What’s your experience been like?