I have probably given my opinion on this too many times. Basically, I do not like the explanation he gives as to why rotating the body sideways and racket elbow and racket arm positioning "must" be a certain way and why everything else is wrong. Actually, his explanation as to the defects of other methods of preparing are exaggerated so as to support his teaching. There is a subtle difference between saying "these things are wrong, therefore this is right" and saying "in my view, I have found that focusing on this can help players as it can help eliminate common problems I have seen when doing it other ways".
I also agree with visor regarding his grip - he talks about a "10" degree shift, but the actual amount he shows is nowhere near 10 percent - much closer to 45 degrees. For me that's too panhandle. And it accompanies the front on hitting action (yes there is some body rotation).
What alarms me is how the things he teaches don't seem to match what I see in the best players in the world. As a basic example, his style of gripping the racket and hitting is completely different to Lin Dan in 2008/2009... and I view that as the best that badminton can be played. So why appear to teach something so diametrically opposed? Another example of a player going completely the opposite in terms of elbow position and upper body position is Tai Tzu Ying - and she is quite clearly the best player in the world with the most deceptive shots, and deception comes from the body turn - so clearly there is no impact on power or accuracy from turning the body in that way. But if you teach everyone that turning is wrong, then you remove these fantastic elements from the game in the long run, which I find unacceptable. God - this turned into a rant and that was not intentional.
And that's basically it - its not what he teaches but the forceful way he misrepresents problems with other techniques to make his point. I can also say that his hitting technique has completely changed over the years - he produced an early DVD called "play to win" which was excellent... but his smash technique now vs then is completely different, even though the principles are more or less the same. I much prefer his old hitting action - it matches what I see now in professionals, and I don't know why he changed.
Rant over. Sorry.