Berserk definitely has to be asterisked, but I think the hardening falls into "bonuses from the same source to the same stat do not stack unless they say so". It is still quite good to very good. Taking a full power Fireball at the beginning of the day gives you AC 24. A bath in lava gives you AC 36 (or up to 53 if you stay in there long enough to maximize damage) and a Greater Fireburst can go up to AC 40. If you can get this ability triggered consistently, combined with the rest of Magic Immunity, and Construct immunities it makes the golem really difficult to assail from any front, except with walls to keep it from the fight. And Stinking Cloud is quite a good ability to have as a free action 1/5 rd, even if the DC seems Con-based, which is less than optimal.
The offense is bad, the skills are bad, but there doesn't seem to be any reason why it couldn't wield a weapon, or play an Unarmed Swordsage and add Heat to its Unarmed Strike. I think I would be willing to overlook lacking offense if hardening was easily accessible. The problem is it's not, and does not last all day when it triggers. I don't see any easy way to deal yourself massive amounts of fire damage except by asking the party spellcaster. As it is, LA-0*.

Damnit, now I'm hungry. There should be an ability that if the golem is destroyed, eating its remains counts as a Heroes' Feast.