I don’t mind the nail smoothing (in fact it’s needed) - it’s the sanding down of the outer wall almost from coronet to shoe that drives me nuts. If a farrier is sanding down a foot, I’m going to take a harder look at the rest of the shoeing job to make sure they’re not just sanding down a bullnose or event lines while not addressing the internal problems. One doesn’t mean the other, but often the practices go hand in hand.
Also, if I’m applying expensive Keratex* to that outer horn because the farrier asked, you’d better NOT be buffing any away! 
*Keratex is formaldehyde based, and changes the molecular properties of the horn in a way that can be beneficial - but those effects only go so deep