yonkyo is painful because pressure is applied to a nerve overlying a bone in the forearm. there are lots of pressure points in the body, but in my beginner's opinion, they all are 'a little something extra' and should not be a focus point of your technique---based instead on the MD behind my name and the fact that i'm one of those folks instructors come to with an evil grin when my nage complains i lack the pressure point. Not everyone's body has read the textbooks, i tell my patients. Those who don't feel the pain that i see in the faces of my colleagues may have the nerve buried, or in a deeper place, or even an entirely new location, or they may have a high pain threshold (their body for a variety of reasons doesn't recognize pain in general as most do), or be on drugs, or last, and most dangerous, may be willing to suck up a lot of pain to accomplish their goal.
in th opinion of someone who can stand there literally for minutes with my hand poised to punch a nage intent on inflicting pain, go for the balance instead.