When did you last try Zulip on Android? I agree it was pretty much unusable in 2017, but everyone who actively uses it agrees it's improved a lot in recent months (I use it every single day, and have a good experience using it to get my work done). The mobile apps remain our top engineering focus area, since they're certainly far behind the desktop experience's level of polish, but we've generally stopped hearing new users describe them as "unusable".
I should also mention that Zulip's threading model makes the experience of replying to a conversation when you get back to your computer feel great, which isn't true for any other chat tool. You don't have to be glued to your keyboard/phone in order to communicate effectively or avoid missing out on participating in important conversations, and I can't emphasize enough how much better this makes life.
That doesn't eliminate the value of mobile (as I said, this is our top focus area right now), but it's a big part of why people love Zulip today, despite the mobile experience being way behind the desktop experience.
¹: https://twitter.com/b0rk/status/986444234365521920