I have never had my nips pierced but I believe it is the most painful of all.
I have had other piercings done a hafada, frenum and PA all retired for one reason or another, these are not very painful at all, nothing more than a pinch.
In fact the stretching to go up jewelry sizes is much more painfull than the piercing itself.
I still have a lorum that is it.
As far as physical stimulation goes there is very little
It is the visual "shock" that gets the motor running.