Interesting, I find that when ukes arm is locked straight in Nikkyo that the technique can be resisted. - Its when there is a bend in the arm at the elbow that severe pain can be rendered to uke with little effort.
If you have good aiki as uke, it doesn't matter whether your arm is bent or locked out. Joint locks stop working with good aiki. Aiki as in the body skill. Course, if tori/nage has better aiki ...