User blog comment:Crimson and Darkness/Shin DxD 3 discussion/@comment-93.135.175.217-20190913173652/@comment-10330822-20190914004551

Perhaps, its still iffy. The problem right now is that he has a hard enough time maintaining Pseudo DxD mode. However, Ajuka hinted that if Ophis and Lilith worked in tandem or fused then Issei should be capable of handling the power, hinting that part of the reason it was so difficult for him to control was that Ophis didn't have her full power.

Also, Issei's soul is being affected by the Dragon God aura, so its becoming stronger and stronger as time goes on, and that might help him control the DxD G mode as well.