Flat UI leads to usability problems. For example see Nielsen Norman group's report on Windows 8 usability: http://www.nngroup.com/articles/windows-8-disappointing-usab... Or read this site for iOS 7 issues: http://uxcritique.tumblr.com/

Now Google is acknowledging this deficiency of Flat design. See: http://static.googleusercontent.com/media/www.google.com/en/... They have a page titled "Dimensionality affords interaction." Google is bringing back some dimensionality because a pure flat UI is harder to figure out.