First let me point out that for some of us the term Hackamore means a bosal and should not be confused with a Mechanical Hackamore that is usually made of steel and has shanks for additional leverage. A traditional hackamore consist of a leather or rawhide Bosal, a Bosal hanger and generally Mecate reins. I would never use a mechanical hackamore, however I have started dozens of colts in a traditional hackamore “AKA rawhide bosal” and have never so much as rubbed the hair off a horse. No different than if I started them in a snaffle bit I would set the horse up to succeed by teaching them how to carry the device and respond to its pressure before I rode with it.
I typically ride all my personal horses in a traditional hackamore through the initial training stages. Sometimes for up to a year or more before letting them carry a bit with a two rein setup.
Just as many have already pointed out the devise is only as cruel as the hands behind it. I see people every year that turn snaffle bits into a weapons just because they don’t understand how to use it or don’t care.
There is lots of good information about the hackamore & bridle horse training process currently available if you are interested.