I've been away several years, but if not wrong this is a subject already touched. Due to my little knowledge I asked my sensei, since according to me it has to hurt.
When I told him that some people here said it shouldn't hurt he agreed at a certain extent. According to him it's up to uke. Sure you can make nikkyo hurt, but if uke follows the technique there is no need for pain. So basically pain gets involved when uke does not follow nikkyo.

And at this point I must say that I can witness it. I am not that great at ukemi, but for nikkyo I just used to go down on my knee and tap. Painful as heck. Sensei taught me that it's because doing that way I, not intentionally, create a resistance, I stop the natural flow of nikkyo. I started going all the way down to nage's side and the pain almost disappeared, it's more a pressure on the whole arm. Sure he can still make it hurt, but as uke I have (or better I should have) the skill to minimize if not get rid of the pain.