Here we are forgetting that also Google was a big promoter of flat monochromatic interfaces for its web applications, and laid out the direction for web sites far more than MS did with its Metro interface.
On small screens, a flatter UI can have some advantages because it uses less screen space (less relevant today because of high res high dpi screens). As long as controls and non-controls are clearly identifiable.
On large PC screens and more complex applications, it becomes quickly less usable especially when mingled with non-controls and becoming ambiguous.
You don't really need a GPU to run an UI, Windows had non-flat interfaces far before hardware acceleration became common for UIs.