I, also, would like to see any evidence of this. I have been doing sute ukemi since I was six years old or so and am now fifty-six. I do have some health problems but I really doubt that they have anything to do with ukemi. My son is now 32 and most likely has more ukemi under his belt since he was five than most people take in a lifetime and has no serious ill effects from it. He works in the medical field and keeps pretty close tabs on himself.
Of course there's ukemi and then there's ukemi. I personally don't do or teach "breakfalls" that create much trauma as the body meets the mat. Most of the force should be dissipated during the roll in the air and there should be very little tension in the body as it makes contact. Timing, no breath in your lungs, etc. and natural posture as you touch the mat is important. When done properly you really shouldn't have much sensation of having "hit" the mat with your body.