I did fine LEARNING ukemi, and I love it. But I seem to have some remarkably weak intercostal muscles, and three months into my aikido training I pulled some other back muscles during a jujitsu/grappling class and that yanked some floating ribs out of place (I didn't know that at the time). A few days later, even though I was doing fine ukemi, on about my 30th roll out of a technique one out-of-place rib whacked another and I wound up with a cracked floating rib.
Bizarre? You bet! But I learned a lot, and met an excellent chiropractor.
We start each class with a series of ukemi rolls, first forward then back.
Now I know that if my ukemi feels odd and my back hurts, it's time for a trip to my osteopath to get some ribs popped back in place. Meanwhile, I'm working on building up the appropriate muscles to hold my ribs where they belong.
It's been three months since then, and so far so good. And no, I don't have osteoporosis; it's usually my connective tissue that gets yanked apart rather than my bones that break, I'm very flexible. And I haven't needed to stop training while recuperating from anything, although there have been days when I had to sit out some things for part of class (or fall on only one side if the other was sore).