In my experience "flat design" seems to have become synonymous with hidden and undiscoverable. Metro and Material were/are horrible in this respect (I don't use iOS so I don't know how Apple's version fares in that regard).
My experience is that it seems to be based on the philosophy of "just touch things to learn what they do" but then you're constantly punished with "oh, you touched something so things changed". It's a minefield.
It's not particularly the "flat" design that's at fault here, but it's lumped with this simultaneous trend of feature deletion and cryptoUI.
Perhaps in theory, but not in practice. One thing about material is that you can't even see the edges of widgets. Sometimes you have to touch exactly on top of some letter because the sensitive area doesn't have any padding. Often when you think you're touching one thing, you end up touching another. Sometimes you have really tiny sensitive areas surrounded by vast wastelands of insensitive negative space with no visual separation of the two. Material also doesn't work well with pointing devices, IMHO. ModernUI seems to have evolved a little better in that aspect. I sense my Chromebook experience devolving with each update.
My experience is that it seems to be based on the philosophy of "just touch things to learn what they do" but then you're constantly punished with "oh, you touched something so things changed". It's a minefield.
It's not particularly the "flat" design that's at fault here, but it's lumped with this simultaneous trend of feature deletion and cryptoUI.