Having used a fast Clay offense for several games this past week, I can say that the biggest disadvantage I found was not being able to utilize Storehouse to unclog my hand from excess good enhancements at the end of my turn (i.e. I could put them in Reserve instead of discarding them, but had no way to efficiently return them to my hand later). One of my opponents suggested using House of Prayer, which I strongly considered, but in the end I couldn't justify using 2 deck slots for roughly the same effect (Storehouse and House of Prayer).