I know Marcus Pettersson is the favored whipping boy this year on D, but consider this:

Since 3/6 (Dumoulin's return), no Penguins D pair has been better at controlling the share of 5v5 on-ice events than Pettersson-Marino. Score/venue adjusted per http://naturalstattrick.com :

They haven't been great at driving offense relative to the team, but suppression has been their game. SVA at 5v5 per hour of play, the Pens are allowing:

8.76 fewer shot attempts
5.46 fewer scoring chances
2.79 fewer HD chances

with them on the ice versus when they're off.

They're also controlling a +5.74% share of the expected goals when they're on the ice. Neither of the other two D pairs even come close to any of this.

Relative to the rest of the league, the results are the same. The Pettersson-Marino pair is allowing 20% fewer expected goals vs. league average and are simply destroying play when it's in their zone. Blue = good below, via http://HockeyViz.com :

Even isolating them, the results are largely the same. The offense isn't totally there (especially for Marino), but they are both above average defenders at even strength (also via http://HockeyViz.com ):

It adds another wrinkle to the overall dynamic on the backend- you have the gifted attackers (58, 5) paired with guys that can handle themselves alone defensively (8, 4) to put teams on their heels, then can deploy 28-6 to be your shutdown pair that can still chip in offensively

TL;DR: Marcus Pettersson and John Marino are the Big Defense Boys and the Penguins are a much better team defensively when they're on the ice together