The Book of Mormon is 531 pages, and about one and a half times the size of the New Testament. It is unusually long for a first-time author. Why would Joseph as a first-time author feel the need to produce such a long book if it was a fraud?
Why would he include so many details about the Near East and Mesoamerica if each one is just one more reason to be exposed? The longer the Book of Mormon, the more chances of being exposed.
In addition to all the details which would need to be factually correct, Joseph would also need to maintain internal consistency. The longer the Book of Mormon, the more chances of being exposed.
If the Book of Mormon was a fraud, Joseph could have made it a lot easier for himself by shortening the book or avoiding so many factual details which could be proved wrong. What reason would Joseph have for needing to produce over a quarter of a million words?