I think school aversion is the heaviest mythic flaw for a character.
Being immune to all benefits from an entire school while also getting a penalty on saves against harmful effects is too bad.
Abjuration: No more dispel magic to remove bad stuff, no more resist energy, no more protection from evil
Conjuration: No more heals, nothing more to add here
Enchantment: No more (greater) heroism, bless, rage, good hope...
Evocation: Admittedly this can be a viable choice. Funny enough, you get immunity to beneficial MIRACLES.
Illusion: No more invisibility, no more blur, no more displacement. Viable but ouch!
Necromancy: This is viable, unless you're undead.
Transmutation: No more haste, no more polymorph, most awesome buffs are here.
In short, of all schools, only Evocation, Illusion and Necromancy are viable choices because immunity to beneficial stuff is too bad.
I ditched the immunity and instead gave a -4 penalty on caster level for effects from those schools, if the penalty results in level 0 or lower, the spell has no effect. You still couldn't be healed by a lowly potion of cure light wounds, but the party cleric can still patch you up with a HEAL spell, albeit with -40 hp.