User blog comment:Jopjopjop/Volume 20 Spoilers/@comment-50.139.232.230-20150718183305/@comment-4320546-20150719025243

Wasn't it because Diabolos is based off of the powers of Ophis and Great Red inside Issei's new  body that SGC didn't work? After all, Empireo Juggernaut Overdrive and Cardinal Crimson Queen are both based off of Sacred Gears. Diabolos Dragon isn't. Of course, since DxD is more powerful than CCQ and isn't based off of Boosted Gear, most likely it is stronger, but we'll have to wait to find out for sure.