Lots of hate for the Ur-Priest and Mystic Theurge
Given the highly effective spell progression of this class, and the relatively low pre-requisites, this is no surprise to me. However, there are two things I would like to note about this class.
There are at least 2 versions of this class. One in the Complete Divine, with Spell Focus(Evil) as a pre-requisite. The other is in the Book of Vile Darkness, with Malign Spell Focus as a pre-requisite. I suggest going with the BoVD version. The BoVD is slightly more powerful (Malign Spell focus is a +2 to save DC's for Evil spells instead of the +1 for Spell Focus (Evil) ). Using the BoVD version also makes the class slightly more restricted, since the Malign Spell Focus requires a pact with some evil power. In any event, the Ur-Priest is probably much more obviously intended as an NPC class.
As for the other point, I would suggest looking past the purely mechanical aspects of some of these prestige classes. The Frenzied Berzerker, going by the intent of the class, is probably never going to be encountered with anything in the way of support from allies. The Ur-Priest steals power from the gods. Something tells me that the Gods are perfectly capable of defending their turf, what with the devoted followers and stuff.
Now getting back to this thread, what Prestige classes do I consider the most potentially broken?
I would put Mystic Theurge right up there. Strategically, there is no reason for a Sorcerer not to take the class, and only a minor dis-incentive for a Wizard (loss of the meta magic feats). However, those that would take this class are already splitting their advancement between Wizard/Sorcerer and a Divine casting class. At level 20, a standard Wiz / Cleric would have only caster level 10 in both classes to use against But a Wiz 3 / Clr 3 / Mystic Theurge 14 casts as a 17th level caster in each class. Its a hell of a choice, either a Hyper Caster with 9th Clr and Wiz spells, or a Nerfed Caster with 5th level spells in each class.
But if you apply the class with a literal mindset, you can only ever have 10 levels of Mystic Theurge, which means at Char 20, you cast as a 15th level caster, getting 8th level spells only.
But yeah.. Trying to stack Ur-Priest with Mystic Theurge reeks of cheese (though i admit to considering doing just that for a BBEG recently).
As a DM, I would probably say that any stacking of prestige class effects is entirely at my discretion. Prestige classes seem to be balanced in relation to the core classes, not in relation to stacking with one another.
END COMMUNICATION