Review Detail of koijiro21 in The Omniversal Chat Group

[SPOILERS PROBABLY] After Triain, there wasn't much use of Oppai's power, look at CxC, DxD, AxA, and fairy wyverns, in fact, after unlocking Triain, Issei began to tone down his perversion a lot until Azazel had to guide him back to the "right" path. The only uses of Oppai's power in the later volumes of the novel are when he used Rias's boobs to gain stamina (which was his predecessors' idea) and the announced engine that will probably transform AxA into ExE.