User blog comment:B214/Question and Answers 6/@comment-99.235.66.221-20161130072057/@comment-9595671-20161214023012

It seems the strength of the dragon effects how potent the poison is and how quickly it can kill them. Ophis seemed to just shrug it off, Vali held on long enough to be healed and Issei stood a while before it killed him.

Though Vali does have the stronger body for now atleast. While Issei can dragonify, I think the reason he only got a small power boost from Ophis and Great Red's power helping to revive him, it's more like being reborn with it and hasn't fully implemented into his phsyiology yet, which probably explains Wukong's training a bit more.

Vali had been a Human/Devil hybrid since birth, so he had most of his life to train the aspects of his heritage. Issei on the other hand has only had the 'heritage' of GR and Ophis for atleast less than a year. Once he becomes more compatible with it, Issei will likely be much stronger without his armor.