Because of the lack of quantity is is difficult for either side to say rock solid, yes NFL HC's are usually successful jumping to college or no NFL HC's are usually unsuccessful jumping to college.
There isn't enough to be rock solid either way.
But, i believe this is completely wrong "evidence that does exist shows that it is not a myth and strongly indicates that it takes a rare individual to make it work."
Sorry, that's just wrong. Show me a list of guys who were HC's in both college and NFL. Break them down by winning % in NFL Vs winning % in college and I guarantee the vast majority will have the higher winning % in college. That does not support the myth that NFL HC's who switch to college have a low success rate. The evidence that does exist shows just the opposite.