Flat design was always atrocious, in that it utterly fails to take advantage of our visualization systems extremely effective autonomous recognition and interpretation of any 3D cue at all.

Not meaning we need things to look gratuitously 3D. But that small amounts of 3D effect, edges, shadows and highlighting, greatly reduce the effort of "seeing" what is where and what it means.

And of course, the trend to simply use text for text, buttons, links, ... without very high standards and consistency of differentiation, is truly horrible design.