Re: does nikyo hurt?
I don't have any objection to it hurting, and it often does, sometimes a lot. But I don't think I'd ever feel safe if I was actually relying on pain to bring someone down. That just seems way too dangerous.
It's such a common experience to have people react to pain by fighting harder against it (especially if they don't know what will stop the pain). In fact fighting harder against anything painful seems to be the most instinctive reaction of untrained beginners - they have to actually be taught that it will hurt less if they go with it. If you surprise or overwhelm them with something painful they'll usually fight it with all their strength and risk injury. But if you lock their joints and use leverage and collapse their body and stuff like that it can work even on an uke who's on their first day, since they don't really need to know what you're trying to make them do.
It's also so so common to ignore pain in the heat of the moment or not really feel it even when you later turn out to be seriously injured. When I think of painful injuries I've had - a broken wrist as a teenager, a toenail torn off in aikido, partially torn tendons - at least 50% of the time I kept going and only noticed I was injured after the adrenaline wore off.
So for me I think it seems quite useful to try to make things less painful and get them working well that way. You can always add the pain later on top if you want... But I don't feel that safe actually relying on pain.