The concept that every message belongs to a topic and the async communication focus makes so much sense to me. I read conversations, not timelines.
It is highly ranked on some platforms that do validated reviews, like Capterra.
(I lead the Zulip project).
If I had to say it would have to be something customizable, letting a user to delete their data even after getting kicked from a server, very fast and seamless joining process ,great gif/sticker support without any premium features etc. But really that's just some fantasy app lol. Discord is doing just fine destroying itself however
By the way, I didn't know there was an instant online test app because when I searched for Zulip I was in the download page and it doesn't say anything about trying it online. Seems like a strange suggestion UX wise but that's how I feel about it (wonder how many people missed out on this?), same thing after you enter the app. It should have a test area for the new user to chat around by himself with a bot or something with locally/session stored messages.
We're planning to roll it out at our company (foundata) in Q4, so you’ll get at least a few bucks from us. I'll also happily recommend it to our customers. As an OSS company and service provider, I can very much relate to the lack of marketing budget and the constant SEO spam.