For me, I would suspect that painless techniques don't provide feedback that provides information to the uke about he technique. I am thinking pain is a means of communication. That communication helps in the learning processes where verbal explanations fail. In the learning process of Aikido we often find direct non verbal communication advantageous. Whole concepts and principles are learned almost instantly communicating non verbally. Pain teaches us limits and boundaries. It provides a framework for learning principles and concepts. We can feel what is happening to the body more accurately and more readily, transference of information, than described verbally. Pain can be an instrument that enhances the learning process, it tell us what is happening as a result of a waza is effective, or isn't.
Techniques that deliberately inflict pain tell me I'm training with someone I'd rather have little to do with. I'm sort of an activist about being maltreated and rarely cooperate with being abused by anyone ranked over 5th kyu. I highly doubt that because a waza is painful it tells me anything about its effectiveness.
I'll agree with a paraphrase of your statement that we get better feedback from feeling many things than if those things are described verbally. But I believe actual pain doesn't have to be part of that process.