so what it does
it CAN cause trauma to the dog. Your dog will choose what is aversive to it. Aversive means something it wants to avoid.
most dogs will not find the stimulation bothering, but really sensitive dogs might
my dog finds the act of me witholding treats from him MUCH MUCH MUCH more aversive than stimming him with a e-collar
it comes down to your dog. but no, doesn't hurt.
can it be confusing? yes. can it be uncomfortable? yes.
but it doesn't hurt. its simply a stim, and i've put my friends e-collar around my wrist and I went up to level 30, waayyy higher than most people use on their dogs. It didn't hurt. it just tighened my muscles and tickled a bit