Prestigious Party

Yeah, basically he would lose the bonus metamagic and item creation feats that wizards gain, and only gain d4 hit dice per level instead of d8 for the Cleric levels.

A Druid 3/Sorcerer 3/Mystic Theurge 10 would lose Trackless Step and Wildshape abilities from the Druid class until 17/18th level respectively if you advanced as a Druid after maxing out Mystic Theurge levels.

A Cleric/Sorcerer/Mystic Theurge would lose the least, just the d4 hit dice from Cleric levels as those classes have no other class features at higher levels.

A Druid/Wizard/Mystic Theurge would stand to lose the most from the use of this Prestige class - Wildshape and Metamagic/Item Creation bonus feats.