One of the biggest disadvantages of the system is that it is totally wrong! The book "Bunnicula Strikes Again" (a book about a vampire bunny written for third graders) has a score of 860, which is the same score given to J.R.R. Tolkein's "Fellowship of the Ring."
The fourth book in the "Diary of a Wimpy Kid" series called the "Ugly Truth" has a score of 1000 while Carl Sagan's "Contact" has 1010.
As a children's librarian in Georgia I have watched this system frustrate parents and turn kids against reading who are told they "can't" read a certain book because it is above or below their score. Dumb!