If you hire someone to write your dissertation, then you are cheating. One of the tasks involved in getting the Ph.D. is demonstrating the ability to report thoughts and ideas on paper in a cogent and knowledgeable way. This is why it is so difficult to be certain that your ghost writer will be qualified: you are not supposed to be using a ghost writer, so it is not something that an ethical person will offer as a service. They KNOW it is cheating, and doing the work for you is assisting in that cheating. Only cheaters will provide that service.