Undead chariot issue.
 

+2 is added to characters armor, without checking is the armor of the chariot is higher.
If characters armor is higher than that of a chariot, this is not an issue. However if the character has no armor or his armor save is low, this become an issue.

Was able to duplicate this with Orks, and Undead.

Ex: Tomb prince with no armor on a chariot armor save appears as 6+ instead of using chariots 5+ save. Come to think of it, this does not even work with 2 added to charachters armor.

Light chariots only give +1 AS (TK special rules).

As for the modification of the character's AS, why would you need to check? The Rules say you use either the modified save, or you use the Chariot's save. The character line includes the modified save and the Chariot line includes the Chariot's save. The Character does not have two saves, he has a choice of using his own save (which can be improved by being mounted) or using that of the chariot.