Actually Godot has a LOT of UI related classes/controls, more than other cross platform game/app builders I've seen (but I've not seen them all). I'm working on a more UI-based (mobile) app than game-focused engines and there's a lot of container controls and general UI stuff that most game engines won't have. They also look pretty good and are theme-able, and work pretty well, with anchoring and stuff. Not so great on mobile but workable, and you can always extend them. Can't comment on the back-end stuff.