For a product that is so simple, there are no good self-hosted alternatives. Mattermost and RocketChat are written very poorly, reliability and getting your data out is impossible.
Slack goes down so often we're thinking of writing a very boring clone that uses ActiveMQ and MySQL, just because chat should be boring and needs to "just work".
I was just considering setting up a Mattermost instance for our company since I used it for a year at a previous job without any issues (I was just a user though, I didn't deploy or maintain it). Just curious, why do you think it's poorly written or unreliable?
We tried running it, so a lot of experience with it, and it wasn't great. It barely stayed online.
For something so simple, you have to run a massive server, like gigs of ram and multiple core, even with a very modest user load. Take a look at the codebase, it's also a mess and impossible to fix any bugs. Finally, if you want to get your data out or report on the message activity, good luck, you'd be better off passing paper notes around. The open source version is nerfed a bit too, no LDAP authentication for instance, so it creates a lot of problems there too.
Slack goes down so often we're thinking of writing a very boring clone that uses ActiveMQ and MySQL, just because chat should be boring and needs to "just work".