User blog comment:B214/Volume 23 discussion blog/@comment-77.188.219.183-20170424022727/@comment-29963364-20170424141912

CxC doesn't reach God-Class (yet :3) because Issei isn't unleashing all of the JD's power (like Vali does in EJOD), but channeling a controlled amount through his Evil Pieces (promoted to Queen).

Like I said before, CxC is a form with better control over it's power usage & output, and therefore a lower stamina consumption rate, and a longer usage time.

Not only that, but the CxC is "incomplete." Issei still needs to fully master using the three forms of [Illegal Move Triaina] so that CxC can fully stabalize.

So Issei's "incomplete" CxC is Satan-Class (if not a little stronger). But when he finally stabalizes & masters it (like Goku & Gohan mastered Super Saiyan in the Time-Chamber) then the "complete" CxC will easily reach God-Class.