Meta-abilities are an emergent property of the Nobilis rules. Can a Power of Serpents create an Aaron's Serpent? If not, why not? If not, what's the closest thing to an Aaron's Serpent they can manage? Unless that closest thing is pretty lame, how is this not horribly game breaking? Why is the War not won by these weapons already?
Law of Hubris makes it viable to say "yes, they can" … an explains why that doesn't tend to happen very much.
Take Power of Granted Abilities. With a Greater Creation they should be able to hand out capital-G Gifts like candy. They should be able to say: "Lo! You are now a Regal of your Domain!" They should be able to say: "New Imperators Spring From Your Footsteps!" These would all be acts of Hubris, whereas granting someone the ability to play piano really well, use mundane magic, or even granting "mundane superpowers" should not be.
Since messing with miraculous abilities has a cost in terms of Hubris, we have an explanation why they probably should not just grant everyone Active Immortality and have Power of Weapons arm half the heavenly horde with Glorious (opposite of Abhorrent!) Weapons.
Law of Hubris is saying "Yes, But There's a Cost", and explains what that cost is.
Thematic appropriateness is probably worth considering when making rulings on Hubris. While using The Sword That Can Cut Through Anything to sever a branch of the World Ash to send a whole world tumbling into the void is probably Hubris no matter what, it is much less gauche if that world was a major beachhead of the Excrucians.
Underlining the horror of that action with a later Actual incursion seems perfectly appropriate, but having that incursion happen right then is probably not the best option.
Whereas, if the branch was cut of because it was in the way, and the Power had had a bad day… then an immediate Actual incursion, or a Riding of Excrucians as a result of the Hubris might be just the thing.