I've heard this argument from others, mostly safety-conscience instructors. A lot of training facilities and ranges around me ban shoulder holsters.
It is very difficult to draw from a shoulder holster without sweeping people with the muzzle.
Back to Jeff Cooper's four rules:
4. Never let the muzzle cover anything you are not willing to destroy.
Now, I understand that the four rules can not always apply (before someone rebuts here), but the ability to observe them with a holstered weapon is always a positive. When a handgun or rifle is in a case and a person walks into a gun store, I can ensure you that the muzzle will sweep multiple individuals (although "unloaded", right? see rule #1). Some people would argue that the four rules only apply when you are handling the gun, aka indicating that you can only "sweep" people if you are handling a gun. I disagree with this.
Now true accidental discharges do happen, albeit rarely. Even if you practice perfect gun handling and your weapon decides to discharge are you willing to accept the responsibility of putting a bullet directly downrange behind you?
Most people also find shoulder holsters vastly impractical and hard to conceal unless you are a very large person.
If you are still interested in a shoulder holster system, you should take a look at the Galco VHS which points vertically instead of horizontally.