During the past few months, I ran an undergraduate computer science research program at my university, and I chose to use Zulip to organize our communication (between 25 people). I wanted to use Zulip because it was open-source and, like you, I was a fan of the threads model. Unfortunately, the participants reported that the notifications were unreliable, the mobile app was janky, and the threads were confusing.

Keep in mind that these weren't average software users but rather CS majors filtered through an application process – even for them, threads took a while to get used to. I concluded that Zulip would work well if every team member was on board with (and understood) the threads model, but a team that doesn't care would prefer Discord or Slack.

Zulip has a tiny change: you have to make a conversation have a point up front by giving it a thread title. random convos can happen in the ‘random’ thread, so it includes the previous model.

That's not a tiny change. That's a huge change. That's the difference between an e-mail listserv and IRC (or a PHPBB forum and IRC). It seems like what you're saying is that you prefer topic-based threaded conversations to the free-form "chat" model. That's totally valid! However the mere act of requiring a topic, in my experience, totally changes the way people approach and interact with the software, and, as a result, changes the nature of discussions that take place. Personally, I think it's a positive change, but lots of people disagree.

Discord recently introduced forum channels that closely approximate Zulip thread functionality, with a much more intuitive UI than Zulip.

My two main gripes with Discord are the default dark theme, and the lack of embedded links.