I would be interested in reading that book. Could you pass along the author or title?
It seems to me that this forum discusses the question of fluency and the limits of mastering spoken Latin quite often. I think some here would argue that Latin fluency in reading, even in the best of circumstances, is quite difficult and rare, and that fluency in speaking is downright impossible today. Of course, it would be interesting to see the trend line of success from 1500 to (presumably) present. You can find many of these discussion by using the search function. I think they will dovetail with the problem you point out.
My opinion, to be taken with a grain of salt, is that low success rates result from the difficulties of learning a dead language in general. Latin is a relatively difficult language and has limited practical application to one's daily life (or at least diminishing as one gets closer to the present). Even though Latin used to be a compulsory subject in most schools, there's no reason to assume that, post-graduation, students maintained these skills, especially, as I said, how little Latin is used in day-to-day affairs. However, other compulsory subjects, such as basic arithmetic and grammar, are used constantly, and thus not forgotten.
On the other hand, it seems that students who choose to pursue studies in classics can pick up basic Latin grammar in a few months, and can be proficient readers in 2-3 years and can maintain (or improve) proficiency as long as they continue to read Latin.
But, I'm just rambling and I do not know what you mean by "success" and what types of students the book mentions, i.e. elementary school students or college level classics students.