Much like "John Q Public" I'm always a bit leery of "-isms", but honestly so long as usability and accessibility are not sacrificed on the altar of "ooh pretty", I'm actually ok with just about anything.

But that's the problem, most of this stuff ends up artsy-fartsy bull that tells large swaths of users to sod off. You can even see it in the examples many such artists under the DELUSION they're designers vomit up. In the process of adding colour and shading back in, they're also choosing colours that violate contrast minimums, lowering contrasts in a way that makes things harder to read or recognize, over-using iconography in a form of false simplicity, or adding back in bloat we spent the past decade trying to get rid of.

In that way though, flat design was the mortal enemy of said "artists" who THINK they know what design is, whilst lacking the engineering knowledge to design a blasted thing. They want all their eye-candy BS back front and center to justify their own existence and skillset; instead of taking the time to learn how to keep their abilities between the safety lines of accessibility and usability.