Horizontal carry can really tend to catch on things, especially in some environments. This not affect us all equally, so some like/dislike horizontal carry more than others. The shape of one's body is a factor, too; some of us have more broad, flat real estate than others.
An alternative is a degree of cant, neither fully vertical nor fully horizontal.
This is just my $0.02.
My local mentor, on things blade-oriented, likes to carry a Strider Airwing horizontally, on a part of his body that has a perfect flat spot to do so. When I try to copy his carry position, the handle catches on things, and concealment is impossible under anything much less voluminous than a Man-With-No-Name-style poncho. (...as worn by Clint Eastwood...) The flattest real estate I have, at belt level, is centered on my spine, which is neither comfortable when seated, nor the safest placefor a hard object to be, if one were to fall backward, and that is before we get into the tactical disadvantages. (This is, of course, the outdoors section, not prac-tac.)
