From my experience with Rocket Chat I'd argue it's about a 30% implementation of Slack when it comes to features and polish. My favorite gripe with it is that

From my experience with Rocket Chat I'd argue it's about a 30% implementation of Slack when it comes to features and polish. My favorite gripe with it is that quickly typing half a username and auto completing will select the first (or some random) user that's doesn't have anything to do with what you typed. It's a real pain if you're a fast typist.