As a dedicated compact user the only thing I can think of that is a disadvantage is when trying to pound with the big dogs the 50/12 combo is not enough. My climbing bike is setup with 50/34 front and 12/27 rear. Works just as well as my triple setup. Sometime you need to double shift to be in the correct gear.
The compact crank is not a new idea. Was floated in the 70/80's and did not take off. More new off the shelf bikes are offering a compact crank setup.