It's a bit complex. Basically only graphic card drivers are a bit problematic. But it's the binary driver compatibility that makes ReactOS unique.

Windows XP and 7/8/10 supports several driver models (incl. legacy) and drivers models are in user- and kernel-mode. So for most devices it's not a problem. The display driver model changed with Vista; Win2000 display driver are still supported. http://en.wikipedia.org/wiki/Windows_Driver_Model , http://en.wikipedia.org/wiki/Windows_Display_Driver_Model , http://en.wikipedia.org/wiki/Windows_Driver_Frameworks