It's a problem of uneven tone.
Much like the phnatom menace, where you have a quadrupole ending, with being sad for qui-gon, then silly kid destroys donut ship, to comic relief with jar jar and so on... It makes the viewer numb.

With the Hobbit, you have the Gandalf segments, which have a very serious tone ala LoTR. It's not only scary, you also get a feeling it is real. Like the physics are real. Such as when he goes to explore the witch-king's tomb. If he was to fall down that chasm he would die.

But then, later, when they escape from the goblin kingdom, it is idiotic slap stick CGI physics.