All that I can tell you is that when I was badged we had to be able to qualify with whatever our previous score was in order to be allowed to shoulder carry. We wore them during the winter if we were doing snowmobile or other backcountry patrol. Thought process being that in order to be properly geared up for colorado winter in the backcountry your weapon would basically be buried. So shoulder rig outside of All your layers. Which is a completely different motion to draw and present. You've spent 10,000 hours doing something perfectly, exactly the same way every time, in order to create a something beyond the concious plane that is automatic and secondary.
That's what you'll do when shit goes sideways.... Down, break, forward, pull, present.... Except the gun isn't there. And while you're thinking "oh yeah..." the bad guy has put two down.
Doesn't matter how well you shoot if you can't get it up and ready before the other guy does.
These days I run a chest rig. Never looking back.