Ad aspera

I thought they had some of the worst passive bonuses. You're always waiting for buffs and healing, and tripled Dreghich checks not only give near-immunity but also increase attack.

Well, let's start with the fact that Fire has only one passive bonus – access to fire magic, and Igg's passive only works in nature. Ignot at least gives the character bonuses to characteristics, has synergy with his own branch (healing effectiveness) and (possibly) some buffs from warrior branches (like the Knight's "Determination"). That is, you can work with him as a warrior (especially with the two-handed weapons branch) or a mage. Brumma, on the other hand, gives not only immunity to two types of damage (although not very common) and negative effects, that is, approximately half of the magical schools, a number of unpleasant abilities from warrior branches, and who knows what else. While immunity to buffs from allies usually doesn't matter.