Seems to be very useful UI. Is Flat Design good for people of all ages and skill level ? I am wondering if such usability testing has been done ? Isn't depend

Seems to be very useful UI. Is Flat Design good for people of all ages and skill level ? I am wondering if such usability testing has been done ? Isn't depending too much on colors cause color blind people to get confused ? How would you distinguish your app from other apps in flat design, won't everything look the same eventually ?