I'm biased, as I lead the Zulip project. But I think this is a reasonable place for me to post some thoughts. Given current events in the USA, I can't emphasize enough how worried one should be about the fact that a few companies like Discord, Google (Gmail), and Meta have databases with access to the private conversations of hundreds of millions of people with their closest friends and family members, linked up with…
I tried Zulip (cloud offering) with some techie/designery friends, so we should have been right at home but... the desktop app on macOS and the web app was visually unappealing and clunky, and we ended up going back to a paid Slack plan.
I looked for docs on how to theme Zulip (so I could contribute), or for existing theme packs that would soften the transition but found neither.
tl;dr: The functionality was good (Love the threading!) but the UI feels like the 2000s came calling. Some UI polish would go a long way.