I am no expert, but I have owned and ridden horses, raised a few and broke them for 34 years now. I have had only a few formal lessons. IMHO, the successful use of a hackamore or bitless bridle, or even the type of bit depends mostly on the skills (hands, seat, leg aids, general balance, verbal) of the rider. After that factor in the experience of the horse and it's personality. Not only training under saddle, but including entire history from birth with or without human interaction. What headgear works on a particular horse for one rider may not be right for another rider. My personal favorite is a tom thumb with slightly long shanks and a fat copper mouthpiece. I am curious about the Cook Bitless Bridle. Does anyone have any experience with it?
