MRoh

Pain can also be caused by uke himself trying to block or resist, it does not neccessarily have to be caused by a poor technique.

To reach the state of "non resistance", uke has to take a lot of nikkyo to make his body "permeable" for nages force.
Nage in turn has to learn to deal with ukes resistance, otherwise a strong uke might push him through the dojo, or if nage is stronger he will break the resistance with his own power, but this often leads to injury.
In a real fight there will be no time to struggle around, but it might be uesful to try something out in a practice-surrounding.

Concerning O Senseis techniques, my teacher said when he was young and he took ukemi for him, no matter where O Sensei grabbed, nikkyo or yonkyo, he felt terrible pain.
Somewhere I read that O Senseis ukes always tried to avoid being grabbed, because when it happened, it was painful.