Flat Design is really "Swiss Design" or "International Style", which came about mid-twentieth-century. The usability issues are typically around lack of affordances – e.g., how do I, as a user, know what's tappable if everything is flat. Using color very intentionally is one approach.

Dark mode is an accessibility issue. Yes, flat designs can exacerbate the issue – white space being a primary design pattern – but having a dark mode version of something would still be desirable, even if we went back to skeuomorphic design.