I had an interesting conversation today with the lady who massages my horse. We got onto the topic of bitless bridles and she mentioned the fact that the cross-under styles (Dr. Cook, Nurtural & the like) can actually be quite harmful and do a lot of damage to the poll and jaw. She is qualified, well recommended and has had years of experience, plus she does a fabulous job on my own horse, so I trust her word for it.
Like people have said previously, the bit/headgear is only as harsh as the rider's hands, but I think it's good to know that the cross-under's aren't the super-gentle alternative people think they are, and can actually cause quite a bit of soreness and muscle damage around the horse's head. Especially if you're riding around with a strong contact - I don't like it that a lot of bitless promoters say they are good if you tend to have hard hands, according to this lady who has treated many horses with issues caused by cross-under bitless bridles, if this is the case you're more often better off with a french-link or oval-link snaffle.
Just thought this was a good bit of info to share.
Do not follow where the path may lead, go instead where there is no path and leave a trail...