Every Domain can be overpowered. For example, Vitality 5, 10, and 15 each drop all Wound and Status Ailment levels by 1. Meaning nothing that wouldn't cripple a person can hurt them, and an attack that would be Massive Overkill like eating a tank warhead as a normal human drops through Overkill, Lethal, and is merely Crippling like losing a limb. Combined with Vitality boosting general durability, and a regular human would shrug off anti-tank fire as if they were in Ubercharge in Team Fortress 2.
Intelligence 5 and so on double, quadruple, and octuple the benefits of intelligence, meaning that Followers would have a minimum of a *8 multiplier to their IQs as of 15, ignoring the boost various Domains add up to. Of course, this would make minor IQ differences more or less insurmountable without spacing into, for example, a type of magic that boosts you.
Dexterity does something similar with, well, dexterity. Finesse, reaction times, flexibility. Dexterity 10 would let you spin your head around like an owl without issue, and 15 would let you spin it like a top. Naturally, this is very useful in combat.