I dont believe there are people that are imune to yonkyo. IMHO imune people havent run into people that actually can do yonkyo. The technique is far more difficult then most people are willing to admit.
I am one of the persons on which a lot of people cannt find yonkyo. About 95% of the people cannt do a hurting yonkyo on me. The other 5% can and then it is like mister Hooker describes it. A dime sized deep bruise and a dead feeling in your fingers (BTW I always shake this feeling away, you know, shake low, shake above the shoulder).
As a side note. I found that my yonkyo really improved from doing suburi with a suburi to.