User blog comment:B214/Volume 23 discussion blog/@comment-31137559-20170412031517/@comment-31137559-20170412114827

Well basically all(well if there are no half-bloods but I doubt it) of the Red Dragon Emperors were human. And yeah, some were most likely killed before they were able to go Juggernaut Drive. But some of them would have had to go Juggernaut Drive because people knew what it was. Juggernaut Drive releases the seal placed on the Boosted Gear and Divine Dividing unleashing Ddraig's and Albion's power giving the user power equal to the Maou and gods(depending on how strong they are, I would say). The user would go berserk and their life force would get used up to fuel Juggernaut Drive. They would rampage until they destroyed themselves with that power. It's basically a final trump card. And it was irreversible. The reason why Issei returned to normal was because he was in an incomplete form of Juggernaut Drive.

Now Issei can be considered the strongest Red Dragon Emperor. But when it comes to the original powers of the Boosted Gear and unlocking them, he would be considered very weak. Instead of going down the original path, he took a different one. I was just saying Issei was so weak because there was his talent, yes. But his very nature went against those of the past Red Dragon Emperors. Koneko even said he was the weakest Red Dragon Emperor, but he was the nicest. Most of the others indulged themselves in power and destroyed themselves with it. Those who had Ddraig's and Albion's power had a cursed fate. And those around them didn't have happy endings. We've heard it multiple times. Issei's the weakest Red Dragon Emperor, but he's the best. We can't use the excuse he was human because it has been stated much later in the series. And even Ddraig and Cao Cao have said it.