That's how it was said. It doesn't seem to be limited to traditional defenses like armors and forcefields. For example, it allowed him to bypass Sacred Gear Canceller that nullifies Sacred Gears and Sacred Gear enhanced abilities on contact. It also seemed like it'd bypass Worthless which nullifies special abilities on contact. Diehauser chose to dodge all of his attacks imbued with Penetrate, but he chose to nullify those that didn't contain Penetrate. I believe Diehauser knew that Penetrate would have bypassed his Power Nullification.
Sacred Gear Canceller and Worthless has no physical manifestation of a defense, if you get what I mean. But I'm a little confused about the ability myself. Issei used it against Thanatos when he was using afterimages. It's like Penetrate would have allowed him to ignore the afterimages and hit Thanatos directly. He seems to use it in melee, but spoilers say that he used it against Vidar and Nyx, so I'll have to check again. If he used it in melee against both, that means it's probably limited to melee range for now.
As for limits, I don't know myself. I'm actually surprised that it can bypass abilities that nullify his abilities on contact.
I was expecting something more of the mechanic that is based the ability. For example, Gae Bolg fron Fate can ignore conventional defenses because the spear conceptually reverse casuality to always hit the target's heart, or just distort space. Penetrate just bypass through anything without much explanation of why.
What you mean with imbued? Is not the ability passive? Or does Issei has to yell to activated?
Yeah, it seems way too broken, and without further definition it becomes annoying.
It's just described as anything that stops him from transmitting attacks and damage to the opponent directly. There might be more to it, as Diehauser was trying not to get hit by it. He's Satan-class and another Satan-class was afraid of his power. Meanwhile, Issei was just Ultimate-class in that form so I don't get why Diehauser was taking care not to get hit by Penetrate.
I don't think it's passive. He just uses attacks and Penetrate happens. He can decide whether to use it or not.
He has always used it in his fights after unlocking it in Volume 18. Rizevim in Volume 18, Walburga in Volume 19, Diehauser in Volume 20, Apophis in Volume 21, Thanatos in Volume 24. Can't remember too much about Volume 22 and 23, but those were concerning Rating Games where strategy was at the forefront.
Well, I think it can still be shown or mentioned since Augusta is dead. Walburga will try to get revenge since she was her mentor and teacher. Maybe she's an orphan or had a bad childhood and Augusta picked her up. Idk, it's just speculation but I hope it will be mentioned.
If her past is mentioned, the readers will feel more empathy for her and understand her actions better. If nothing like this happens, Ishibumi probably has no major plans for her. In this case, my only hope would be that she's released to Slash Dog.
I know, its just that two of them can be the reincarnation of the lady or princess and the guy from the story being reborn. Walburga insanity can be attribute to the princess going crazy in the very far past after she had made a decision of killing her lover.