As mentioned above, shoulder holsters are not popular, and there is good reason for it.
For CCW, unless you carry a small framed, short barreled, light weight gun, and are always wearing a jacket, and preferably weight over 200lbs, you will print - bigtime.
For range use, most ranges will only allow verticle drop holsters, which are awkward to use.
They are not allowed in PPC, nor I suspect IDPA. IPSC used to allow them, but the last freak who used one in Canada now wears high heels and makeup. I'm pretty sure s/he isn't the reason IPSC banned them, but you never know.