Ok, admittedly the Arcane Heirophant is more powerful than the Mystic Theurge... by a very large margin, even. That's because it takes the aspect of the Mystic Theurge that makes it over the top, then adds even MORE useful class features to it! There are no drawbacks to this increase in usefulness. The Arcane Heirophant's existance doesn't make the Mystic Theurge any better balanced against other spellcasters: both of them STILL gain the full spellcasting benefits of two separate base classes, which are balanced against other casters mostly by virtue of their strong spell lists, minus 2 spell levels in one of the classes. Both prestige classes offer no significant drawbacks to this huge boost in spells usable per day and spells available for use.