To be fair, at this point it is just a matter of a small sample size and luck. Pacioretty's line concedes a lot of odd-men rushes against and spends quite a bit of time in their own end. Right now they are benefiting from a high on-ice SV%, but the chances against are still there. Last night's DD OT turnover was case in point.
They've been bailed out or have just been plain lucky which has masked a lot of the flaws of their usage and two-way play.
That's part of it, but another part of it is the fact that they haven't been nearly as bad defensively as they get criticized for on these boards. People just remember things like the odd-man rushes and ignore the good defensive plays they make.
They also easily spend the majority of their shifts in the other team's end. Part of that is zone starts, but part of it is that line (especially Pacioretty) is very good at driving possession and getting more chances than they give up.