I'd say the biggest problem with relying customer ratings is that once customer's learn how valuable they are, some customers will exploit it to try to get more

I'd say the biggest problem with relying customer ratings is that once customer's learn how valuable they are, some customers will exploit it to try to get more than they deserve. Without consistency the ratings are pretty worthless. You need someone who will honestly rate the employee based on the value they provide to the business, and the business is the only entity that can do that.