It was the the largest browsers in Indonesia and India, at least in part because it's marketed as saving $$ on data by blocking calls to download ads and other scripts[1][2].
Sure, lots of people install custom browsers because they don't like the standard ones, myself included. Fennec (Firefox but from f-droid) is super slow, Chrome is not an option for privacy, but Lightning[1] is fast and has most of the features I want so that's what I use at the moment. It might be vulnerable for something, but I'm taking my chances with some lesser-known browser versus having the pain of a super slow browser all the time. This is also one of the reasons I don't have a banking app installed or have ssh keys on my phone or something: my phone is just not as trusted as my laptop.
I remember that UC is one of the ones I looked at, probably even installed to try it out.
Edit: sibling comments mention popularity in India or Asia, but I'm from the Netherlands. My brother also has a habit of finding custom things (independently from me), sometimes quite questionable apps... it's not just asians that install non-google apps.
Yes. The userbase has grown bigger more recently due to the browser's free in-built VPN being used to circumvent the government's ban on ~800 porn sites[1].
That only represents the data of Google Play. Considering China is one of the largest markets of UC, yet doesn't have access to Google Play, the actual number would be a lot more than that.
According to iResearch[1], it has 311m "monthly unique devices" in China in Feb 2019.
[1] https://www.scmp.com/tech/article/2130205/alibabas-uc-mobile... [2]https://marketingland.com/alibabas-uc-browser-beating-google...