I dislike how spells are tied to almost entirely to shards. Meaning the "free" amount you get is generally never very good, and you need many shards to make the spell potent.
It ties too close the size of your empire is the strength of your spellcasters. On top of that, there appear to be two game creation options which affect spells. One being the obvious, choice to affect how strong magic is, but then the "Resources" option, if it affects amount of shards on the map which only makes sense that it does, means it as well affects how strong magic is.
Also are there any cases where a physical based hero can actually be diminished in power outside of killing them? If you get attacked and start losing shards it actually starts reducing the power of your spellcasters, whereas physical heroes remain as strong as ever.