Is it a valid comparison if they're comparing sentence length of translations of works originally written in other languages? For example, Hermann Hesse's Siddhartha and Plato's Republic.
What does "valid comparison" mean? If the intent is to help potential readers to estimate the difficulty of the book, then that's useful.
Couldn't the original degree of difficulty of a book be altered during the translation process? Not sure if this is an apples to apples comparison. If you stick to books written in the same language, then perhaps.