I do occasionally, but it didn't take me long to find differences in behaviour and support between Linux and iOS. Entire APIs are left unimplemented on the Linux side and things that work on Linux break on mobile for some reason. Codecs (for image, video, and audio) seem to vary wildly between platforms too.

I'm sure Apple could take Gnome Web and turn it into a cross-platform Safari browser if they wanted to, but so far they haven't (and probably don't want to).