In my opinion when you have people telling you and warning you about horses running through bosals and loosing control you have people that don't know or understand bosals. Yes, they can learn to run through them but if used correctly they are great tools. They don't get the refinement that bits can offer but for hacking around and trail stuff they work well. They do limit your showing outlets a bit because of the age limitation that are set up in the shows but if you don't show, no big deal. I love my hackamores and now have moved up to the two rein on my gelding.
When I got my first hackamore at the local tack store( it was junk but it was a starting place) I got the same story. Be careful, don't ride the horse outside an arena, hell run away with you , bla bla bla. It lightened my horse up dramatically in short order from the snaffle he was started in. There's a quote from Martin Black I think. "If you want to improve your horse try changing the bit, if you want to improve your horsemanship try changing your hands" the hackamore forces you to do that.