Just to clarify, when a tetori uses their iron body ability, it impairs them as though they were wearing heavy armour? I can't find any mention of an exemption to the normal effects armour has on monks, and it seems strange that the ability would have such a steep built-in handicap. The immunities and resistances are great, though a monk who is 19th level is probably going to benefit more from their AC bonus being stacked than mitigating a relative pittance of the damage they're likely to incur without it. Intentional trade off, or am I just overlooking an obvious exception?
