+s & -s of shoulder holster rigs...
I honestly would not call vertical or horizontal shoulder holsters the "best" carry systems or the first choice for many handgun models but some important factors may require this method.
I strongly disagree with many gun shop employees, security class instructors or sworn LE officers who quick to rail on shoulder holsters in any form or for anyone w/o knowing-understanding things like health, strength, duty or uniform position(aircraft pilot, 911-security dispatcher, vehicle driver, etc).
Many cold weather LEOs, armed guards and CWP holders may need to use a shoulder holster for a BUG/second gun.
Now it's true, the firearm could be hard to re-holster one handed or it may require a awkward draw but it's not fair to knock shoulder holsters for EVERY gun owner/armed professional.