Illusion is a much worse mess, though. It's a school of magic that affects enemies by level in a level-scaling game that doesn't tell you the enemies' levels (unless you use the console, which I have done), every Illusion perk (and I do mean every Illusion perk, except for Novice/Adept/etc.) is simply a way to un-nerf Illusion so that your spells don't bounce off of everything, and eventually you're forced to dual-cast almost every spell (and/or use a strong potion) just to effect enemies that are the same level as you.
Illusion is the strongest school of the five when fully peaked. The master level spells are a joke. But the expert level ones when dual casted, will affect, in a small radius, almost any creature or NPC.
The very small and exhaustive list of unaffected creature or NPC are:
1. Dragons (including Alduin)
2. Graybeards
3. Children
My experiments with summoning hordes of various monsters, in master difficulty, to challenge my min-maxed dual-wield smith-enchant-alchemy Breton has lead me to the conclusion that fighting 50 ebony bow wielding drauger death overlords would be extremely difficult or mayhaps impossible without illusion spells.