Did I get it right that modesetting DDX and xwayland aren't affected by this
problem because xf86-video-amdgpu technically differs in some substantial
aspects?

Apart from FreeSync (which is triggered by pageflipping, thus disabling
pageflipping doesn't help at all as a workaround), it would be bearable if
modesetting DDX was usable. But then there is this bug, which forces the user
to play everything with vsync, which is not always desirable either:
https://gitlab.freedesktop.org/xorg/xserver/issues/629