Shoulder holsters tend to be floppy, and they also put your gun potentially more in view and definitely in reach for an attempted gun grab. View wise, they’re closer to eye level and more likely to be noticed if your jacket flaps open. Because they aren’t really anchored to you, it’s harder to incorporate retention mechanisms that will help keep the gun in place either against a drop or a grab. It also means a longer and slower movement for the draw- across the body, and probably sweeping a later arc with a loaded firearm to reach your target. Because the shoulder holster will be hanging and moving at least a bit, it could be less predictable where, exactly, the grip of the gun is to be found. A proper belt holster on a stiff leather belt mounts the gun solidly and predictably in one place, gives the shortest and most direct draw path where you shouldn’t lose anyone between you and the target, and because the holster is pretty much fixed static you have a reliable ability to disengage retention mechanisms, usually activated with your thumb. Also, with a belt holster, I can put my badge directly beside it and closer to my belt buckle so that anyone who does get a glimpse of my gun should also see my badge. In Canada at least that helps reduce people getting freaked out.
There might be occasions where a shoulder holster is more comfortable, but that’s about the only advantage I could think of. If true concealability (versus simply being discreet) is the priority, there are better options than shoulder holster.
