I think that there are 'flat' interfaces and undifferentiated interfaces. TWM, Windows 1, Windows 2, System 7 (in fact, all pre-MacOS 8 MacOS) are all Flat - but perfectly differentiated (and, to my eyes at least, very clear and elegant)
Windows 3 - Windows 7, macOS 8 - macOS 10.9, iOS 1 - iOS 6 are all differentiated and 3D (to my eyes they're very clear, but also a bit gaudy)
Anything recent, as far as I can tell, other than some special Android / Windows skins or Linux variants, are flat (which is elegant) and undifferentiated (which is sadly unclear).
So don't have a downer on flat UIs. Have a downer on undifferentiated UIs instead!