One of my biggest issues is that "flat design" by definition also dictates "borderless". Meaning, a dialog or other window adjoins or overlaps another, you sometimes need to stare for a few seconds to tell where one window begins and another ends. Seems to be worse on Windows and Chrome, but I see it on Linux sometimes too and try to file a bug when it happens.
Adding at least a 1-pixel border and drop shadows to all windows can go a long way to fixing this, but I don't believe that the latter helps "dark mode" users much.
(Of course, a lot of developers these days only use and develop apps and websites in full-screen mode on a tiny 13" laptop screen, so they never see a lot of the issues that "flat" causes.)
Adding at least a 1-pixel border and drop shadows to all windows can go a long way to fixing this, but I don't believe that the latter helps "dark mode" users much.
(Of course, a lot of developers these days only use and develop apps and websites in full-screen mode on a tiny 13" laptop screen, so they never see a lot of the issues that "flat" causes.)