A big reason that NSSA, NMLRA, NRA ban loading from flask or horn during competition is because of how their competitions are held. There are 25, 50, or more competitors on line at the same time loading and firing. The possibility of a stray spark from one of those competitors causing a horn or flask to explode is infinitely greater than at a CAS match. When I was competing with muzzleloaders, I always loaded from pre measured charges when firing in relays; but, still used my powder horn and and antler tip measure when I was the only one on the firing line.