It's not uderrated at all, it gets just as much praise from its fans as the other FF games. The reason why some people dislike it is mostly because of the cartoonish and doll like character designs and the world it's set in. FFIX stepped away from the innovation of VII and VIII, and returned to a more traditional look and feel. The story is set in a mostly medieval society, as opposed to VII and VIII's futuristic settings.