I'm in the industry in so much as my father has had race horses in the past (of varying abilities!) and I've got a couple of retired racers which I ride. My best friend is a retired trainer (who I don't ride). My passion isn't jump racing, I stick to what I know which is low grade all weather...
I haven't any exact figures, but I'd say as a general rule that the harder the ground is the greater the danger to the horse and rider. Less give in the ground on impact.