The way I have so far experienced and understand it. Wrist, and other joint, locks work just fine without pain having to be there. that's why they are called locks. Ideally the pin locks up the joints in such a way as to make it difficult if not impossible for Uke to move. If Uke does attempt to move and resist the pain is actually applied by him, to himself, and not by nage.
Then let me ammend my statement. The thinking is that properly performed aikido techniques need not be painful to be successful when performed in a dojo on a properly compliant uke?