There are many things that can influence pressure point susceptablility, alcohol and other drugs are big ones, emotion and adrenaline can also make a huge difference. For example, most wrist techniques have very little effect on my, my joints and bones will start to shred before I ever reach a pain threshold that would immobilze me. On the other hand, my elbow joints are very sensitive to pain and nerve points.
In our MA classes, we were instructed never to rely on only pain-compliance or nerve-point techniques for this reason, their failure rate is far too high to depend upon in a serious situation. My brother, who worked as a professional bouncer and was trained in martial arts, backs this up. He has come across more than a few drunks/crazies that seemed to have an immortal resistance to the typical techniques taught in police training or general martial arts.
We were taught to always go for a mechanical joint lock in preference to a pain-compliance lock. Mechanical locks will work every time, if the guy resists and escapes it will only be with his joints/limbs damaged or broken as a result. Relying on a pain-compliance lock will get you in trouble.
The best method IMO is to use pressure points combined with mechanical locks, it throws some extra butter onto the bread of your self defense. e.g. an elbow lock combined with your fingers digging into those special gettin’-to-know-ya spots.