Thanks for the feedback. It would be really hard to actually put a ban on it as it's the natural back up plan while the victim is defending the rear naked.
But the concern remains. The risk is higher then in other techniques (outside the other cranks mentioned), IMO. The "everybody would tap" idea is very subjective. A fracture can happen in an instant while you believe you'll be Ok.
Did you see the blood spurting from Story's nose? Tht indicates that along the crank, the blood flow in his veins was being constricted as well, increasing the blood pressure in his brains. He could pass out before tapping. The natural tendency in a perfect rear naked is the attacker to continue the squeeze as the guy goes limp while in the crank should be to continue the crank. Scary at least.