On an upright bike you've got a little bit more time and you have better angles to control the fall, but there's also the reflex to turn your skull away from the point of impact, which is really late in the fall. Of course it can happen, but it needs more than just a fall, some freak occurance that somehow slingshots the cyclist to the ground or something. I saw a video of a teen guy on an upright who got hit from the side by a car on a 20 mph road, and he ended up on his feet, with the bike still between his legs, and cycled off (it was his fault, he should have given his name and phone number).