The way I always understood it, the term "hackamore" referred to any bridle that was bitless and you would use terms like 'mechanical' or 'bosal' or 'sidepull' to describe the type of hackamore.
For instance, I would say "I just started riding Dobe in my bosal hackamore" or "I am considering buying a mechanical hackamore".
As for the difference, a bosal is a more complicated piece of equipment and can take years to learn how to use properly. Used improperly, it is fairly easy to either skin a horse's face up or teach them to brace and ignore it. It places upward pressure on the horse's jaw and downward pressure on the nose with a bit of poll pressure in there as well. With an experienced person, it can be a fantastic training tool for young horses.
Mechanical hackamores, on the other hand, are often misused and I am of the same mind as KV. I don't really like them either just because it is so common to see them used improperly. They are not really designed to give clear cues if you try to direct rein and they can be exceptionally harsh. Basically, anyone who has a horse that neck reins well and travels on a loose rein and the rider has soft hands can get along okay with a mechanical hack but IMHO, there are much better bitless options out there.