[wittyphrasehere]

This.

"Flat UI is bad for usability" keeps getting repeated as a criticism against the flat UI trend but is not actually true.

Bad UI design is bad for usability, regardless of VISUAL style.

As jack_trades mentions, there are MANY other ways to convey interactivity without artificial depth, texture.

Think about a simple modal. Users know to look at the top-right, sometimes left, for controls for the modal. What difference does it make if the user sees a flat X icon or an X inside a beveled, drop-shadowed button?

Re: desktop vs. mobile; If a user has to hover over an element just to know if it's interactive then perhaps the design has already failed.