Dummy spurs have a very small surface area that makes contact with the horse's side, therefore, contrary to popular belief, the dummy spur is actually quite severe.
I use a Sprenger disk spur, with a 250mm shaft. The disk is smooth edged and spins, therefore when applied, it covers a great surface area and also rolls along the horse's skin rather than jabbing into it.
Obviously, all spurs should only be worn by a rider who can control their lower leg. And should never bruise or cut the horse. If this is occurring, you are abusing the use of a spur and should go back to learning to ride before trying to train a horse with the use of a spur.