The issue is support for older hardware and that is the manufacturer's responsibility. It wouldn't be surprising that, for example, a video card mfgr not write new win 7 drivers for an 8 year old video card (after is “discontinued”). Now if you can use the xp drivers in Win7 then it should be okay. But that doesn't always work as promised.
My old raid mother board might not have the needed drivers and will probably default to some generic functionality.