The cognitive load it's trying to guess where the button lies in the interface for flat screens. Not an issue under GTK2/3/4 with Zukitre (and QT5/6 reusing it with qt5ct/qt6ct or with an environment variable setting QT_STYLE_OVERRIDE to "gtk2" or similar.
I remember running into issues with link-style clickable text in some parts of the UI that was almost unidentifiable as clickable because it stood alone with no similar text in non-link style nearby for reference.
There were other issues that I clearly remember. There was some remarkable jank when moving the mouse cursor across screen borders in a multi-monitor setup. If you were moving towards the edge of the current screen, Windows would under certain circumstances trap the cursor there instead of moving it across the border. I believe this was done to give "hot corners" a bigger mouse target, but that feature was almost completely DoA on desktop.
I didn't remember having issues finding buttons on windows 8.
While they were certainly flat, they were always clearly signaled from my memory - did other people have this issue?
To be clear, when it was released I was one of the people hating on it, but it grew on me over time - and after I installed startisback, which essentially just scaled down the start screen/Metro ui to a slightly larger start menu ... It was a decent UX again, to me.