Zulip is such a joy to use after other solutions. Threads are something that's hard to bend your team around at first (mainly non tech users), but they seem so obivous afterwards and it's hard to use another team chat without them.
One of the few things I wish it had was better admin and audit tools, because in my industry that is mandatory and while you can do it by poking the database yourself a reliable, featured tool would be great. As-is I use Zulip for internal chat and another solution for external (with customers).
In general, in its control panels and settings and options Zulip maybe feels a bit too "power user, then we restrict" than the other way around, eg when having an external user on my network it's a bit overwelming that their settings screen includes the entire org settings (but they can't change them). Pushing to your team is one thing, but when pushing something to customers or 3rd party any unwanted complexity is a major pain point.
Frankly in general Zulip doesn't LOOK like something you want to show to the people you ask money from (your customers), just like your car dealer doesn't want you looking at the inside of the car's engine. I know it's a difficult complaint to tackle because it's not about functionnality but purely perception.
Hope the team does well and looking forward to the future