Completely agree... some people have extremely thick forearms and while you are busy pressing away to find the nerve cluster, he will probably escape... more important would be whether you are connected to uke's center via the yonkyo lock to off balance him
Worse than escape, attack you!
I was told also some people just don't feel it.
..a crazy extreme would be the fact that some drug users don't feel pain at a certain point. Not saying I'm fighting off meth heads left and right, lol... just saying I think relying on pain compliance isn't doing your aikido justice.