I’m an iOS user so can’t speak specifically to Android. The mobile app experience isn’t as strong as the desktop browser experience but works reasonably well.

I will say that the mobile app experience has been improving very rapidly so it’s going in the right direction. For a while the poor mobile experience was Zulip’s biggest drawback but they’ve made huge strides.

It's unusable on Android. Messages arrive days late at time and other mission critical issues. I have contacted the Zulip team about it and they acknowledged Android app still needs work, web is most stable.