It is a rubbish idea that just looks like a laser printed page. Fine for content and totally rubbish for menus, buttons, navigation or interaction. NNG have done other tests and been saying this for ages.
I've worked on GUI design since 1980s and it's definitely purely a presentation aesthetic, very poor for interaction. I still find Windows 10 "control panels" / "Forms" totally baffling in terms of what is informational and what is interactive/clickable.
Even Linux Mint with Mate desktop has some "flat" applications creeping in from Gnome3/Unity/Ubuntu.
People copying things because they LOOK clean is a curse, as is excessive glass/smoke/blur/shadow and Skeuomorphic. Simple clear obvious icons, text on a button (using language resource file if the function isn't super obvious. Very simple 3D effect of two bright lines and two dark lines.
People imitating the worst "koolaid" from MS, Apple or Wordpress themes is mental.