I’ve been talking about the inherent problems with flat design for a few years now. (Affordance anyone?)
We finally have some evidence that flat design is harder to use:
The results show that a search in flat text mode (compared with the traditional mode) is associated with higher cognitive load.
Source: (PDF) Flat Design vs Traditional Design: Comparative Experimental Study
Twice as hard in-fact and much more error prone:
A search for flat icons takes twice as long as for realistic icons and is also characterized by higher cognitive load. Identifying clickable objects on flat web pages requires more time and is characterised by a significantly greater number of errors.
Here’s a video showing the problem well. This is a user new to flat design and well, you can see how he is struggling: