I have an ATC and can carry in the field.
A shoulder holster is sometimes suitable for field carry depending on conditions.
I carry my Ruger SRH .454 Casull crossdraw in a Mernickle flap holster on a nylon 2.25" garrison belt.
During the bear hibernation season I can carry a smaller, lighter handgun like a 4" .357 or perhaps my GLOCK.
Shoulder holsters tend to be uncomfortable while wearing a pack but the good thing is that they keep your handgun high and dry.
I agree that shouder holsters and cross draw rigs make it possible to sweep others at a range with your muzzle during the draw. I always practise with this in mind.