It depends on the horse, the type of bit, and the type of bitless. Soda goes great in a rope halter, ok in a SJ snaffle (getting better), and hates the bitless. Flame goes great in a TWH bit, ok in a snaffle (better now that I've improved my riding), and appears to like the bitless (Dr. Cook). I may try him in some type of bosal at some point in his training.
I have a friend and ALL of her four horses are ridden in bitless. Two of them go in a Dr. Cook bitless and the other two in a raw-hide type bosal. Another friend has used everything from a raw-hide type bosal to a mechanical hackamore to a twisted wire snaffle to a curb. All on different horses.
I'm a firm believer of not backing yourself into a corner with your equiptment choices or training methods. Figure out what works for you and your horse. Don't stick with a bitless even when your horse shows you repeatedly that he/she doesn't like it. On the same token don't use a harsh bit to control a horse that lacks training. Seems pretty darn simple to me. Use common sense and think about what you're doing and the real effects of pressure that are caused by different devices.