The infant version has a lot of very convincing rhetoric that it's the only way you'll ever get any sleep, and both the infant and older kid (toddler, child, teen) books insist your kids will be little brats if you raise them differently. Those are very real fears that our selfish and isolated culture magnifies.
I have never felt so exhausted as when we were trying to follow Preparation for Parenting, not even when I was tandeming, and so much of GKGW is the antithesis of how Jesus taught us to treat brothers and sisters in Christ. My kids aren't perfect, but they are sincere about trying to behave without being locked in some antagonistic battle to get them that way.
I hope that other study isn't as bad.