The only thing in the Mythic Adventures book that has me scratching my head thus far is the Spellcasting sub-ability within the Intelligent legendary ability you can apply to legendary weapons (Page 172).
-It's unclear whether you can "save" unspent points to use for a level 6-9 spell, or if the intent is to cap them at level 5.
-It's unclear if spells with expensive components are permissible; even capped at 5th level, you could put raise dead on it, then be ahead 5k gold each use... Or permanency, for arcane casters. (OR, if players CAN 'roll-over' the points to get 9th level spells, Wish and Miracle...)
Those are both things that I can easily houserule... I'm just curious what the design intent is. All in all I have to say it's definitely worth the money, though, my players are loving it so far.
If anyone has any light to shed on that, it'd be appreciated.