If you're talking about a mechanical hackamore, they are not
gentle unless you have really soft hands. Allow me to re-post something from another topic;
Originally Posted by dressagexlee
- It utilizes three means of pressure, the most prelevent being downward acting poll pressure (which tells the horse, "Head down."), front-to-back force on the nose (which tells him, "Head in.") then the conflicting upward acting chin leverage (which says, "Head up.").
- It works completely off of a curb "bit", meaning that you are constantly using leverage and poll pressure without escape. I've actually seen people attach a secondary "snaffle" rein to the joining part of the hackamore "bit". However, I'm not sure of the effect exactly of doing this, but I'd assume it works completely by putting pressure on the hard part of the nose, which is not good.
- In the care of a rider that doesn't have soft hands, these conflicting aids often cause a horse to evade by curling his head into a low and deep position (meaning that he lowers his head so that he is overbending, and sucks behind the vertical), locking up the lumbar spine and thus eventually producing some of the problems that surface from LDR ("low, deep, and round", a practice similar to rollkur/hyperflexion, but less extreme).
I don't know anything about other types of hackamores, but I often seen people underestimate the force of the mechanical hackamore, or bitless bridles in general. Just because it doesn't have a mouthpiece doesn't mean it is gentle and can't be harsh. Harsh is in the hands of the rider, but a mechanical hackamore is not something I'd consider for a young horse, or a rider that isn't gentle or giving with the hands.