Butt shot giver here too, and that is on the advice of my Vet. Less chance of sniffness causing the horse to stop eating, most times, it is easier to work it out in the bum.
I do have one mare, who honest to goodness, you can NOT get a needle in either her neck, or her butt. Her, I give the shots in the "big" part of her chest, and she doesn't even flinch. Anywhere else, and that darned needle will BOUNCE right off her body. Only thing I can think of is that she was badly "shot" as a youngster prior to us buying her.