I know this is not intended to be a discussion about the merits and faults of shoulder holsters but since I've spent more recent time in one than anyone else I know I'll just point out that in my opinion the problem is not flagging yourself or others; in fact I found that easy to train around.
The problem is that if it goes hands-on you have this thing flopping around, and your draw stroke is really easy to foul. If your clinched up with someone, getting to your gus is probably easier for them than you. Defending it is hard. If you go to the ground it's everwhere and possibly pointing at you while getting slammed around.
So while I do like them for certain applications, the entangled gunfight in a shoulder rig is really, really ugly.