Looking at the rainbow servant, I see that you need to be at least 5th level to get in, and it grants +6 caster levels over 10 class levels. So at 15th character level, you'd be casting wiz and cleric spells as an 11th caster, effectively 2 spell levels behind. If you then took mystic theurge all the way to 20th level, you'd be 16th caster level for wiz and cleric at the end.
I still don't see how it's broken.
I still don't see how it's broken.