Why does the ludicrously ginormic mace of LOTR's Witch King work on the screen while Alluda Majaka's sliding horse won't?
Even if it's pretty hard not to notice it, the main reason is not good or bad FX, but context and plausibility.
The mace looks and behaves like a massive piece of metal, the wielder looks and behaves like some sort of malevolent supernatural being, therefore the silly mace doesn't make us laugh.
On the other hand, the horse and its handler... well.. I think they speak for themselves.