There is deep knowledge which the book scratched the surface of that is under-appreciated/misunderstood to this day.
A sure sign of pushing the envelope is that not everyone will agree with you. In fact the vast majority will think you are wrong. Ironic, considering the results Hogan has achieved.
I used to be guilty of dismissing Hogan's 5L as an old-fashioned tome that is surely out of date in view of the progress that modern knowledge of the swing and technology has made. I recall hearing Sir Nick making the 'battling a hook' and 'created a nation of slicers' comment on golf channel.
As I picked up the game, I made my usual rounds of magazine articles, various golf books, videos on youtube, etc. Thought I built up a pretty good library of knowledge about the swing. I could discuss accumulators, geometric oriented linear force, D-plane until the cows came home.
Until I picked up 5L and studied it in conjunction with working on my swing. Wow. What a revelation. Thought I knew the golf swing pretty well after all the study and work I'd put in before hand, and yet, this small and thin book still blew me away.
Personally I guess in a way I should be thankful I had listened to the naysayers and put off discovering this book until I had gathered enough knowledge and went down enough dead-ends to fully appreciate how profound this book is.
I also agree with the point made in some previous posts, it's not an easy pick-up and fix your swing band-aid despite its deceptive and sublimely understated brevity and wonderfully done illustrations. In contrast with for instance, GD articles which are designed to target the masses with limited time/energy to devote to the subject. And yes, it is a tightly 'coupled' method, to use a software writing term. As in, it is not easy to isolate and implement chosen elements and expect concurrent gain.
But in my humble opinion and my personal experience of the book thus far, to those who persevere, remarkable insights and discoveries await.