Off topic so forgive the digression, but Kobayashi Yasuo Sensei is in his 70's and still takes koshinage ukemi, as well as doing some of the most effortless suwari waza I've seen. I'd say I have to agree w/ Abasan, about the Ukemi bit.
I'm also a bit confused by the varying definitions relating to IS & AIKI. are they the same, related or different?
Well, I've met too many, read the stories of too many, felt too many teachers and students with wrecked bodies to agree with you. Shoulders, a mess, backs a mess, knees a mess, a couple with hip problems.
If you want to argue percentages, it still doesn't change the model, the percentages would tell you that repeated falls -no matter how good you think you are-are not good for you. Then again, so says every PT and sport therapist I know, and for some strange reason the ones who were damaged know as well.