Fantastic Beasts should have been a great success, but I think it failed to learn the lesson that the Harry Potter series should have taught Warner Bros. about JKRs stories
The stories behind JKRs characters are very complex and detailed, with story arcs that stretch across much more than you'll ever cover in a movie. The full story needed to be written down, or at least broken down into book size chunks and THEN edited down to movie length.
By having JKR as the only person who knew the full story, the movie makers had to trust she was making the best edited version of her story- which I don't think she did effectively. JK held on to too many story elements and scenes that should never have made it to the big screen.
The Audiobook to Movie length comparison shows us that you need about 12 Hours of reading to make a good 2.5 hour film adaption, and then you also need a great film editor to work out how that story is best interpreted for film.
