Flat design is a crutch for mediocre designers
From Jonathan Libov
In short, flat design makes it easier for all designers to do decent work, and much more difficult to do amazing work. ...
But if you're an untrained, not-so-great designer like me, please relish this era of flat design. Whenever the pendulum swings back to more ornate designs, or whenever animation quality becomes the primary differentiator between good and bad apps, it's going to become really difficult for an untrained designer to pass as a decent designer.