In this guy's case, it wasn't even flexibility. He just never felt the pain - it was odd. He was somewhat resistant to what you're calling a z-lock (for us, "First Wrist Technique" - quite descriptive, eh?), because he had that lack of pain response and was relatively flexible AND had very strong joints. If you got it on all the way, you had him. A bit off, and he could press out. I've never met anyone else with that combination, nor anyone else who could press out of that lock so easily, so I'll call him an outlier. I have met a few who it was difficult to get on (to an extent that I simply would choose something different), because their joints were so strong and tight (and their hands were big). If it got on 50% they were done, but getting even to that point was less dependable than with most people, unless I caught them in a really loose transition.That's unfortunate he broke his hand. I was asking because wrist locks have fine details that makes them hurt so much more. That's why I asked MA what locks in particular has he experienced and what the person's background was that applied them
I agree that some people are more flexible and the effectiveness varies from person to person. But have you encountered people that aren't responsive to a z-lock or Nikyo lock? Im not suggesting it's impossible, just curious.