Writing this on FF for iOS, the biggest reason I want to use the Firefox engine is for Firefox extensions. WebKit doesn't support extensions except the App Store ones in Safari, I want to use traditional extensions from AMO. Just let me use uBlock on my phone!

I do agree, the Firefox iOS UI is clunky, but I find it useful for stuff where I want to sync passwords or tabs. I use Safari for browsing the web casually because it's nicer, and the feature of swiping between tabs is so convenient.