Drawing from, or replacing into, the horizontal carry is no more dangerous than drawing from any other holster, AS LONG AS you maintain good trigger discipline.
Well, that's not QUITE true. Remember, we follow ALL FOUR rules because they provide the layers of safety that make living with deadly weapons safe enough to practice.
So what you should say there is,
"Drawing from, or replacing into, the horizontal carry is no more dangerous than drawing from any other holster, AS LONG AS you maintain good trigger discipline, and you don't point your gun at yourself or others at any time!".
(Yes, once it's safely tucked away and your hands are off of it, then you can assume it is a safe gun and not exercise extreme caution about the muzzle. Guns don't just go off when holstered. But sometimes they DO "just go off"
in the moment of being handled and holstered.)
There ARE ways to safely draw from and holster in a shoulder or cross-draw holster, but they take a level of attention and technique that's higher than strong-side holsters require because you DO have to sweep the gun through an arc around you (or move yourself through the counter-arc) when orienting the muzzle to the holster mouth. Instead of simply ensuring that the space in front of you (that you can keep your eyes on) is clear, you have to be aware of, and clear, all the space you'll cover with that arc of swing. Otherwise there is a rather high potential that you'll pass your muzzle over folks standing around you and behind you (and even your own weak-side arm) while still handling the weapon.
Just because your finger isn't on the trigger DOES NOT MAKE THE GUN SAFE.
1) Treat all guns as always loaded.
2) Never let the muzzle cover anything you are not willing to destroy.
3) Keep your finger off the trigger until your sights are on the target.
4) Be sure of your target and what is beyond it.
You CAN'T ignore Rule 2 just because you think you've got Rule 3 protecting you. ESPECIALLY in a moment of heightened risk, like when holstering the weapon. (That's when a LOT of negligent discharges happen.)
...
This is one reason shoulder holsters get a raised eyebrow from range staff, and are forbidden in competition. It isn't that they CAN'T be used safely, but that their safe use isn't intuitive and a whole lot of shooters have never really thought about what they're doing.
Hypothetically...
You ensure you're facing a safe backstop, pick up the gun, give it a visual check, load, top off your mag, press check, safety on -- all safe as can be -- and then
sweep that gun around and point it at everyone on your left, at your own arm, and then at everyone behind you right at the proverbial "most dangerous moment of your day" (that moment of shoving your gun into a holster). Whoops!
I'm sure none of us here do that sort of thing, but because the heightened safety requirements are not intuitive, and because new shooters (or old ones) rarely have someone walk them through the proper technique, this is far too common.