In any case, you haven't yet established why pain in the application of the technique is bad.
If the point of nikajo/nikyo is to control uke's knee through the arm, and if this can be done without pain, then a pain stimulus may disguise poor technique. If all you're interested in doing is making uke drop to the floor then pain may
be adequate (although see above comments about performing nikyo on the unsuspecting and seeing pain without the desired compliance). I tend to think there is more to aikido then that.