I've found the bitless bridles that affect the nose and poll of the horse only works when a horse is well trained in how to respond to those particular pressures. With most forms of pressure, especially on a sensitive area like the face, most horses push into the pressure trying to push past it. So when there's pressure on their poll their likely first reaction is to shoot their head up to push past that pressure. If you teach them, with just a flat halter and lead rope, how to put their head down with pressure on the poll first they'll know how to relieve that pressure appropriately. Same for giving to pressure on the nose, but most horses understand this, given that's how we use a flat halter.
I personally love sidepulls that just engage the nose of the horse, they can of course, be made more and less severe with the use of knots, firmer/softer materials, some cross under the chin all in all, you need to find the one your horse is comfortable packing and train him to respond to it.
The biggest concern with anything that crosses under, whether engaging the whole head or just the nose, is the immediacy of release, so stick with softer materials and metal rings.
The other form of bitless are ones that have shanks, the longer the shank the more you needto neck rein. I've seen some little tiny shanked hackamores which could use direct reining, but would be strong. While the true mechnical hackamores with real shanks do need to be used for neck reining only, direct reining will just twist the noseband in all sorts of confusing positions for the horse.
Another important thing to remember big time, just cause it doesn't have a bit doesn't mean it can't hurt. These are tools that operate with the use of pressure and release and all provide some form of pressure (in varying degrees) so be sure to be clear in your signals, consistent in your releases and train your horses well enough to respond to the lightest pressure, saving the firmer pressure for true emergencies. :)
That's my 2 cents! :P