User blog comment:B214/Question and Answers 6/@comment-26926813-20161218101805/@comment-29963364-20161218104813

The [Juggernaut Drive] is the "forcefull unleashing " of the Heavenly Dragon's power.

The [Empireo Juggernaut Overdrive] and [Cardinal Crimson Promotion] are the "controlled accessing " of the Heavenly Dragon's power.

Vali himself said that, while he can temperarily power the [Juggernaut Drive] with his Demonic Power instead of his life-force, he can't actually control it. He can only enforce some of his will onto it. It's like trying to drive a car at full-speed, with no brakes. You can try to force it to turn slightly to the left or right, but you can't slow down or make sharp turns.

The [Juggernaut Drive] is the Heavenly Dragon's power running amok. Vali and Issei can't control it's power output, it's speed, or anything about it. The best you can hope to do is shut it off before you hurt someone, or yourself.

But with the  [Empireo Juggernaut Overdrive] and [Cardinal Crimson Promotion], Issei and Vali can "access" the Heavenly Dragon's power based on how much they can handle. They have complete control and awareness of their actions and how much power they use. It's like taking water from a water tank. You can only carry how much your cup can hold. Vali's "cup" is just bigger than Issei's.

In Vali's case:


 * His EJOD can "reach God-Class" because he's controlling his power output to a level that he can safely handle

In Issei's case:


 * In his CCP, he has trouble against Grendel and Euclid (who are Ultimate/Satan-Class) because Issei can only handle that level of power at that point.