Being functional to some extent, flat design is still thought to be a less intuitive type of design, though. The truth is, the excessive flatness of the objects on the screen can get users all mixed up (especially, if it comes to those of us who are not used to web or mobile interfaces). As I said earlier, clickable elements could be easily confused with non-clickable icons and texts, which may result in a much poorer user experience. So material design tries to introduce some skeuomorphism back nevertheless, it does it in the most simplified fashion possible. It can look really flat (especially, when it comes to colors), but stays multidimensional thanks to the Z-axis approach which is always taken into consideration.