I think maybe it's a very difficult control to nail.
I agree. And since those people who supposedly do not feel pain from this technique, probably haven't been uke for every single sensei around the world, there is no way of really knowing...
The fact is, that regardless of why it doesn't work on everyone everytime, it is better to go for the shoulder. IMHO it's easier to do, and not as risky for nage.
PS. I won't quit trying to get it to work, though!