User blog comment:B214/Volume 22 spoilers/@comment-24583889-20160621010300/@comment-24.113.252.213-20160714200139

Also, in Vol. 8, Issei's Evil Pieces were close to malfunctioning because they were exposed to Ddraig's full power when Issei activated the Juggernaut Drive. Ddraig is a God-class being, so your statement about Gods being to powerful for the Evil Pieces to work on them may have been correct, but just seconds later, Ajuka adjusted the pieces to handle Ddraig's God-class power.

So we know the Evil Pieces have the potential to work on Gods, but the Gods still have "Systems" that they are centers of, and need to maintain. So just like how the Biblical God's "Heaven System" was almost wrecked when he died, the other Gods can't, or shouldn't, be tampered with.