One thing to consider, which is has been danced around in this thread, is that a coaster doesn't fit Universal's current design philosophy. Especially in a hyper-themed land such as Nintendo or Harry Potter, an exposed coaster is an immersion breaker whether you see it from the land itself or on ride. This is especially true when you look at the effort to either hide the ride system (as in the case of Forbidden Journey) or to integrate into the story (as with Gringott's).