I first heard about this 3 years ago when I was taking a class, and I’m glad I didn’t bring my Uncle Mike’s vertical holster with me when I was taking the class, but it really wasn’t that style that was an issue. Understandable so, shoulder holsters, if not properly used, can be a hazzard to people behind the firing line, especially if it is a vertical rig. Everyone seems to think of the Galco holster The Miami Classic when they think of that style of carry, but not all holsters are like that. They look cool, but standing next to several people with that style of carry isn’t going to be safe when everyone is holstering and un-holstering.
I do not think that shoulder holsters are less safe, and I personally think they are very practical for people that do a lot of driving, and it’s one of my favorite ways to carry a large size semi-auto. Paddle holsters and OWB holsters are hard to draw from in the seat position and Bulldog holsters along with Uncle Mikes make very inexpensive holster rigs. The Bulldog holsters have a double magazine pouch which gives you a little bit more weight distribution. Shoulder holsters aren’t the best for CCW because the straps can ride up on you, but if you wear the appropriate clothing they can work.