Flat UI is also really difficult, as it can easily look cheap, boring, and unfinished.
Even Apple’s initial move to a flat UI in iOS 7 suffered from this. It took a long time to get refined to the point of not feeling like a major step backward. I still preferred the look of iOS 6 to anything that came afterwards. The skeuomorphic designs were warm, inviting, and fun. They served as a nice juxtaposition to the rather austere hardware.
Even Apple’s initial move to a flat UI in iOS 7 suffered from this. It took a long time to get refined to the point of not feeling like a major step backward. I still preferred the look of iOS 6 to anything that came afterwards. The skeuomorphic designs were warm, inviting, and fun. They served as a nice juxtaposition to the rather austere hardware.