My general impression -- which I couldn't really give any evidence for and is probably incorrect -- was that there was some degree of genetic compatibility which would determine whether the person would ever begin training, but it was a combination of, as you said, personality, will, rigorous training and possibly sheer luck that they finished it without mind-lock or similar. I've never had much of an idea as to the frequency of mind-lock, however; I've always imagined it to be not especially uncommon, but despite it having no small degree of significance to Kyber I've never considered any kind of probability of it happening. That said, it seems reasonable to assume that it is far less frequent than it was at the capsule's inception, if only because of alterations to the training process and so on. If I recall correctly, the pilot hadn't had any prior specialised training with regards to the pods before being plugged in, either -- unlike capsuleers today.
Likewise, I thought that success was pretty much measured as either 'you can', 'you can't, despite genetics, for any one of a number of reasons' or 'you can, but then get mind-locked'.
As I said, these are all more or less assumptions that I have made; I couldn't justify most of these with any solid evidence.