well I like to think that the DOA characters are made of kevlar or witches, an Izuna drop 20 ft in the air off a cliff spinning at several hundred miles an hour would smash anyone's head like a watermelon. including Hayabusa himself just a small Izuna in a room smashes heads in NG think what an Izuna from a 200 foot drop would do.
Same can be said with the guilotine throw Busa literally pulls off heads with that throw in NG yet again in DOA to make things "fair" all it does is just wind them a bit, not to mention that Busa is technically invincible when he dies and even THAT takes some doing a cut throat/stab through the heart barely leaves a scratch, Ryu can come back to life through the spirit of the falcon I mean Ryu dies in Ninja Gaiden 1 before chapter 3 of the game that's the plot of the story.