he only reason syllabus/books would be unsuitable for a class is if the
students haven't been accurately level tested, in which case it is a problem with the
class, not the books or the syllabus.

There are many reasons any syllabus and specific textbooks would be unsuitable for a class. Accurate level testing is not a solution to any of them.
Text books and testing are profit centers for the business of education. The most common syllabus flaw is that 'educators' completely ignore student individual objectives for language learning, confident that they know what is best for the students. These teachers polish their one-size-fits-all syllabus, administer their standardized tests, and sell their favorite books. Their language learning philosophy is capitalism.