All abilities that can change your stats regardless of what they are presently (for casters who don't have the physicals to be fighters normally), can grant you ~20 points of extra armor, can grant you ~10 extra attacks, can grant you any creature's special attack, can grant you any creature's modes of movement and can grant you autosuccess on many disguise checks is going to be pretty much automatically broken.

Fact is that until Polymorph-type effects are fixed to depend on the character's own stats, be unable to grant 12 attacks, limitless stat boosts & natural armor and any attack modes and modes of movement, they're going to be by far the best at...just about anything. Simply because they can do so much. They are actually pretty hard to fix and they need it bad, so yea.