Is this score adjusted? A lot of the reason the raw numbers don't look terrible is because of score effects. The Devils have been playing from behind a lot, needing to push, and the opposition fine with sitting back.
Per NST for the last 25 games, I see Devils between 49% and 52% for all of Corsi, xG, SC, and HDC - which is okay, but not great. But that is not score adjusted. At Evolving Hockey, which has score-adjusted stats, the Devils are 50% Corsi but down to 47% xG. That is not good at all - ranked 25th in the league. And it's been trending down.
I think there is some evidence that Ruff's systems are essentially being "solved". I've been pretty consistent that I don't like Ruff's playing style, as basically no modern NHL teams play a defensive zone scheme that the Devils do. Video below from Jack Han also has some interesting hypotheses on NJ's systems flaws. The breakout fails are things I've visually noticed quite often. The forecheck is something I have not visually noticed, but running an aggressive 2-1-2 forecheck is something that basically no other team does now.