Ukemi is bad for your body. Period. Learning it is smart, continuing to taka falls your whole life is not good for your body. It will not teach your aiki. It will not teach you internal power, you do not need to fall down to "feel your teacher" and learn the waza.
You can learn power and aiki and how to use it, without falling down at all. And in the process some of the damage will be reversed through training the body correctly.
I continue to prove all of the above, (and not just talk about it) ...month after month as I meet so many of the damaged teachers and players.
Well, the very nature of the question defines the lack of understanding of what it is we do. Taking lateral loads is nursury school and just one of many benefits of proper training. It is far deeper than that and broaches the topic of the connection between internal strength....and Aiki.
See that's where I think differently, but I don't have facts to prove this just observation.
1st, ukemi is bad for you only if it's done improperly. I think people with good ukemi can last their lifetime on the mat without getting injury and pain. It's just when we make mistakes and go rigid, or resist, or try too hard that we get injuries. Relaxed ukemi and we can see quite a few old sensei's taking this still, allows them to train well without pain even today. Movement from the center and a relaxed body almost always result in better ukemi. And we can see this not just in aikido, but in Systema, in kobudo, in wushu, in Silat. And I don't know for how long, but in Parkour you see crazy ukemi that did not just come over night, but through intense physical training and good technique. Anyone doing what they do without those two ingredients would certainly get injured if not fatally, quite seriously.
2. I still see internal strength as different to Aiki. In my limited exposure, I feel internal strength as 'internalized power' which is atypical of Chinese soft arts. Whereas Aiki as something which occurs from outside. In effect, internal skill is you and Aiki is others...