if the visual design is affecting usability, there's something wrong with the execution. there's no aspect of "flat design" that necessitates removing visual affordances of hierarchy and interactivity. many designers take it too far.
this works both ways as well. overdone skeuomorphic designs often look too interactable, as in the old twitter example here. the entire header looks like a raised button. it confuses the purpose of the header and adds visual noise.
if the visual design is affecting usability, there's something wrong with the execution. there's no aspect of "flat design" that necessitates removing visual affordances of hierarchy and interactivity. many designers take it too far.
this works both ways as well. overdone skeuomorphic designs often look too interactable, as in the old twitter example here. the entire header looks like a raised button. it confuses the purpose of the header and adds visual noise.
regardless of aesthetic visual style, you've got to get the fundamentals right or else it's shitty ui design. i wrote about this a couple years ago. http://jim-silverman.com/blog/skeuomorphic-vs-flat/