A poor polish job ruins the guns value, and can make it illegal, as a lousy polisher can actually "Pull" the serial number, manufacturer's data, etc. from the gun during polishing.
Can also dish out screw holes, ruin screw heads, pin heads, any opening into the gun, formerly flat surfaces, formerly square edges, I have seen all these screwed up by polishers who didn't deserve the title, or the customers money.
Always ask to see examples of a gunsmith's custom work , bluing, polishing, coating of any kind, stock refinishing and modifying, etc. before spending tons of money on a valuable, or sentimental value, firearm.