User blog comment:B214/Question and Answers 6/@comment-99.235.66.221-20161130072057/@comment-10330822-20161214032733

That is what I meant by Vali being more balanced. Issei doesn't have stuff like strong demonic power and talent in magic that Vali has, but his raw strength and durability are roughly equal to Vali's. This is what Sun Wukong was talking about when he was training them. When it comes to things like raw strength, Vali and Ise are equal, in fact, he suggests Ise may have a slight advantage, but Vali is stronger in different categories, thus making Vali more balanced. Basically, Ise is more of a "power maniac", while Vali is more like your typical High-Class or Ultimate-Class devil.