Organization of discussions is outstanding, awesome searchabilty, very very fast with low CPU and RAM usage. Never had an issue, using it daily for 4 years in our remote first organization.

LaTeX & Code rendering. Sponsored Pro plan for academic teams.

I really like the enforced structure - conversations are organised into channels (top level) and then topics (lower level), which is a really nice way of organising things, which Slack and Mattermost don't have. In both Slack and Mattermost you have threads instead, which is fine, but I feel like there needs to be a strong group norm on using those, which in my communities there does not seem to be. Zulip provides a slightly firmer nudge to seperate out different conversations. It is also possible to move conversations into the right place if people start them in the wrong place, which I like. It's also possible to set your notifications more finely in Zulip than it is in Slack or Mattermost, which I really like. I also like that there is a free hosted option, and since it is open-source it would be possible to move yourself onto a self-hosted open if the community were to grow and you didn't want to pay Zulip. They're also generous in providing free premium accounts to open-source and research projects. There's also a very active support community (which uses zulip as a forum chat, of course!) which seems more responsive and grass-roots than the Mattermost forum. I'm excited that there is a matrix bridge, although I haven't looked into that properly yet.