NLP is not really a therapy, Ken. It was devised as a technology of modeling excellence, which purports to help people access the effective strategies of others.
The founders of NLP modeled Erickson. They did a good job of extracting some of his patterns of excellence. In their early work they used the Transformational Grammar of Noam Chomsky as a tool. I believe they could have added immensely to psychotherapy, but that did not seem to be the outcome that they had in mind.
Subsequently, NLP has become a technology, somewhat of a paint-by-numbers approach. Although flexibility is extolled, I do not see it in practice. In modern NLP, there seems to be an emphasis on rote procedures: Run patients through a proscribed symbol-drama and they will change.
Erickson was a master at utilization, using indirect methods and tailoring practice to the unique style of the individual. NLP pays homage to these methods, but does not maximize the potential of these orientations.
NLP has become part of popular culture. Many NLP trainers have no standards for what they teach. They will teach hypnosis to salesmen, homemakers, lawyers, etc.
I have learned from reading NLP theorists. I especially like the work of Robert Dilts, David Gordon, and Steve and Connirae Andreas.
You have asked a "big" question, Ken I have provided a little answer. I hope it stimulates replies from others. A valuable debate could ensue...