Technically, or at least hopefully, all the affinities should be balanced. They're all useful in different situations. Neither should be "stronger" or "weaker" than another. Cyro Cross can potentially deal the most damage out of the four area-effect spells, but you need to be in the center of an enemy formation to get the most out of it, since it centers on the Hero, as opposed to being able to blast things from afar, like with the other elemental skills. Pyro Hail has the smallest area-of-effect, but also the lowest cost, etc. While it's true that they may end up unbalanced, it's still possible to run into something that's resistant to that element, in which case its usefulness diminishes greatly.
With that in mind, I do admit that Shadow skills dealing less damage does bug me and does seem to make things a bit unbalanced.
What would be interesting is if other characters had affinities of their own; that way you'd be prepared for multiple situations, instead of the main character being the only one with elemental skills.
Poll