Foe-Hammer isn't a mono sphere card in any way...
Of course with the added depth comes a lack of breadth across multi-spheres. As with all deck building, it's a balance.
There is no ballence in a mono-sphere deck.
As I though, I understand that the mono-sphere decks need to be at least viable and able to be used if you wish.. I just do not see why anyone would choose to. The very nature of them restricts you to only a sinlge deck, and single play style. You have no flexability in your game... it is just the exact opposite of what deck building is about, witch for me is building a deck of answers.