Funny, I was always taught to use the radius for yonkyo and specifically not to aim for the nerve. The reasoning given was that if you get used to having the pain of stimulating the radial nerve aid in the throw you will not be able to do the technique well against somebody with a high pain threshold. Of course, my sensei also said that should you happen to find it without looking for it...
As far as nikyo and sankyo being painless, I have been thrown using both those techniques without anything more than a mild stretching, no pain at all. I don't know if I've ever achieved that myself, it's a little hard to tell on the throwing end. Especially with nikyo I feel it when nage breaks my balance, then uses the focus against my forearm to drop me into his center. This, of course, would only work if uke was trying to attack with the other hand, but that seems to me to be the point of the technique anyhow.
It also seems that any time spent finding the pain in a technique is a lovely time for uke to dot your i's and cross your t's.
Just my opinions of course,