From my own experience I've learned that I can't rely on pain compliance as the cornerstone of a technique; it can temporarily aid in mental kuzushi, but without physics and biomechanics working for me (read as: gravity and commutative locking of joints), I can never get a noncooperative uke to drop.
Example: I was messing around with a friendly, relatively experienced (brown belt) BJJ player a year ago, who had no real experience in aikido other than what wikipedia told him. At one point during a clinch, I managed to somehow get into a good yonkyo grip on this forearm and was well situated in his blind spot, so he was pretty much at my mercy. I cut down sharply like I was taught, cutting like a bokken, and expecting to send an electric shock through his system and propelling him down onto the mat...
... and he just screamed bloody murder, hopped a bit to regain his balance which had been thrown forward, and yanked his arm out of my grip. There was a bruise slowly forming on the inside of his wrist, and he said his entire arm was tingly. I apologized profusely, and he said he wasn't angry, just that the pain was surprising, considering that he had always heard that aikido was a "gentle" art. (that made me snicker inwardly)
I explained to him what yonkyo was. Interestingly enough, he said that when he felt the pain, his first instinct wasn't to go down in order to alleviate the pressure: his first instinct was to yank his arm away, and that propelled him forward enough to be able to receive the momentum I was imparting into him through his arm, and so he kept his balance. He said that, if anything, the pain was a signal to him that I was doing something and his body went into automatic defense mode.
That made a lot of sense to me: most people don't think that falling on the floor is the best way to defend themselves against anything. Their first instinct is to remove whatever is in pain away from the source of pain, like yanking a singed finger away from a hot bowl.
This piqued my curiousity, and his as well, so we experimented with yonkyo some more, him remaining completely noncompliant and moving in ways that are completely not the way aikido uke's move. He did all sorts of things: he yanked his arm out of my grip again, and when he couldn't do that anymore he did some funky sutemi technique involving some bizarre flip and arm-barred me. He told me that he wasn't intentionally countering my technique simply because he knew it was coming; he could FEEL it coming whenever that tingly pain started running up his arm, and he acted accordingly.
Then I realized that even though I was trying to cut his arm down like a bokken, I wasn't: in order to get all that pressure bearing down on his pressure point, I was holding his arm in classic yonkyo grip:
It was perfect for applying lots of pressure into his radial nerve, but thats not how I was taught to hold a bokken: my hands were too close together. There was no control, and no leverage. All the power I was imparting was going into bruising up his skin and shocking his nervous system as opposed to affecting his center of gravity.
As soon as I moved my grip to more resemble a grip on a bokken, with my lower hand on his wrist and my upper hand on about the middle of his forearm, there was nothing he could do to prevent me from cutting him straight into the mat. He also said he felt less pain on his forearm, though he still felt some. He said he felt as if his body were being whipped around more this way than the previous way, which makes perfect sense considering I had increased my leverage and really managed to cut like with a bokken this time. He also mentioned that he had no warning that it was going to happen: without the pain as a signal, he couldn't prepare himself to regain his balance in time or counter the technique.
As a reward for being such a great partner, I returned the favor: he worked on getting past my guard for the rest of the session (I was more flexible than his training partners, and also didn't move like a BJJer, so I presented a different challenge, or so he told me).
Anyway, it was a great insight into the technique, and also a great lesson that not all people react to the same stimulus in the same way.