During my Judo days we did every which way ukemi, there was no need for unbendable arms. Didn't take a lifteme either.
I'll jump on the bandwagon. I just don't get the "it takes a lifetime" for choose your isolated concept/principle/technique. Sure there is always room for improvement and perfection is elusive but it really does not take much to understand most. It gets easier to pick up new stuff with more experience but I am far happier talking months even weeks rather than years. Frankly it sounds more like self-congratulation rather than reality.
With respect to the unbendable arm (this should be minutes rather than years) what differentiates principle from a party trick is when you use it as part of your technique. The story was that I was already Shodan before I was shown the unbendable arm while visiting a Ki society dojo in Canada. Jaimin mentioned te-katana and yes that was exactly what I knew it as - just didn't know it. We (as in Shodokan stylists) see it in the the-katana dousa and the show-te awase (pushing practice) done every session and we do those with an implicit understanding how they apply to the execution of technique. There is no question that it is a principle.