It is not clear from the text of the Polymorph subschool nor from the text of specific spells in the Polymorph subschool whether or not type changes.
For example, if I cast Monstrous Physique, does my type change to Monstrous Humanoid? Am I immune to Hold Person? The spell specifies that you can gain subtypes (such as aquatic and amphibious), but those are very specific. Beyond that, it only says you "assume the form" of the creature of a particular type, but it is very unclear whether or not you gain that type.
Benefit of not gaining the type means spells such as Enlarge Person still work. Drawback is that spells such as Hold Person still work. But, it is not clear which way the rules intended.