As for Milner's training style:
A lot of "knowledgeable" retriever people don't like him because he doesn't use an E-collar and he doesn't force fetch. Those two right there make him anathema to a lot of "knowledgeable" retriever people.
Further, he is very much in the British style of training. Again, a lot of "knowledgeable" people don't think much of British training methods. For example Milner doesn't think a dead straight line out and back into freezing water is always the best way to retrieve. If running the bank is faster to a shorter swim and less tiresome for the dog, he's quite happy with a dog running the bank. This again earns him the disrespect of "knowledgeable" retriever people. British training handles to the area of the fall and then the dog is expected to use his innate talents to find the bird. This again is pretty much out of line with American methods used by "knowledgeable" retriever people.
Lastly, he got into clicker training. I'm thinking that is not a well accepted method for "knowledgeable" retriever people.
I will say I don't agree with all of his precepts but I learn something from everyone I train with or read their books or watch their videos. I think we all find our own style of what works for us.
YMMV.