Flat design is bad UX because it took a useful and familiar dimension, simulated depth, out of the toolbox. I agree that simulated depth was over used and that

Flat design is bad UX because it took a useful and familiar dimension, simulated depth, out of the toolbox. I agree that simulated depth was over used and that the minimalist aspect of flat UI did improve aesthetics in many cases, but it was a net loss due to the UX problems it caused. The right thing was to do depth aesthetically correctly (only a few pixels are needed, and only on important elements) while still retaining the usability advantages of it. Unfortunately the design world (and the art world more generally) appears to be extremely prone to binary thinking: either something is obviously wonderful, or it must be discarded completely.