I disagree, they’ve got a consistent UX framework across the board, inputs are clear, navigation is the same across gdk apps. Is it consistent with other DEs? Not quite. But all gnome apps are easy to use, have pleasing UIs and generally share patterns that make it easy to see them as part of the same family even if an app is third party.
Eh can’t really blame it for not being more open I think to customisation, it is an issue but not really a UX one I think. Any UI could be faulted for that then, not being customisable enough. As for apps not written for it again, not something they have control over. Could say the same about any DE, or even Mac or windows when they use non standard blocks