As my favorite CO put it, many years ago,
"People often say that Safety is our Number One priority. That isn't true. If Safety were our top priority, we would put all these aircraft in irons. Our number one priority is Mission Accomplishment. If we go about accomplishing our mission in a professional manner, then safety will be a resultant benefit. Professionalism is key."
He was exactly right.
So, for those who say that shoulder holsters, et al, aren't the safest possible rigs for carry or for practice, that's all well and good. But, if the mission dictates that those holsters are the equipment to be used, the two options become to err on the side of theoretical safety (IE don't train using the holsters), or to err on the side of professional mission accomplishment (IE train using the equipment one will be issued, but put safety measures in place after conducting proper Operational Risk Management).
I'd put it to you that carrying a weapon in a manner with which one has not trained, is setting up for a very unsafe condition. Just ask the USAF personnel who were killed by the Afghan colonel in Kabul last year if they agree... then again, you can't, because he killed all of them before any of them could draw and use a weapon.
If "instructors" are only training people for shooting basics, static position, at a range, then "absolute safety" is an affordable luxury. If instructors are training people who realistically might need to use those weapons, then real instructors find a way to accomplish the mission, professionally, with proper safety procedures in place.
Those who do otherwise are poseurs.