Doesn't have to get shots on net
Doesn't have to cover for his partners mistakes
Doesn't need to make breakout passes
Doesn't need to separate opponents from the puck
Doesn't need to clear the crease
Let's go one by one.
The shots on net thing is a terrible argument. Matt Niskanen, who we can all pretty much agree has played well, has attempted 49 shots in this playoffs. 14 of those have hit the goalie and registered as shots on goal. The remaining shots were either blocked (21) or missed altogether (14). Basically, Matt Niskanen has missed as many shots on net that have actually hit the net (not counting blocked shots). My point is that saying "Oh, ______ can't get shots on net" is a terrible criticism. I'm not absolving Scuderi of not being able to hit the net, but complaining that he can't isn't really saying much since one of our better shooters only hits the net 50% of the time, when he isn't getting the shot blocked. Also, Rob Scuderi isn't and never has been a shooter. If you're expecting Rob Scuderi to be the one to shoot, I'm sorry that I have no answer for that, but you should probably re-evaluate your expectations for Rob Scuderi in the offensive zone.
He should be able to cover for mistakes (and he has at times). Not sure where you're getting that I said he doesn't. Regardless, I find it really hard to blame Rob Scuderi for getting stuck in a 2 on 1 because Matt Niskanen decided to leave the middle of the ice open for Carl Hagelin. Not to mention Hagelin's shot placement was near-perfect. Not much you can do about that. If anything, Scuderi helped cut the angle down to the point where Hagelin needed a perfect shot to score (and he did). There really wasn't much Scuderi could do there because he had to respect the other guy across the ice. So again, I find it hard to blame him there, but that won't stop people from trying to blame him.
This falls back to the relying on him for shots thing, but Scuderi is not a breakout passer. He is a defensive defenseman. If you're relying on him to be the one to start a breakout, again, you may need to re-evaluate your expectations. He was like this in 2009 as well, so this should be nothing new to people here. This is also why he's been paired with people who can at least move the puck a little (Letang).
He separates opponents from the puck with his stick, which I touched out. Players don't need to make a hit every chance they get. I keep coming back to this, but if you're expecting Rob Scuderi to be a Kronwall or Orpik and start hitting people, you may need to re-evaluate your expectations. He's never been much of a body guy. His strengths, even going back to 09, have been his stick and positioning by forcing guys to the outside. Hitting just has not been a big part of his game. You'll see it from time to time, but you shouldn't expect it all the time.
Never said he didn't need to clear the crease, but find me an instance where a Penguins defenseman has done this recently. Singling out Scuderi for that is laughable, because no one else does it on a consistent basis (not even Bortuzzo). Like with the hitting, Scuderi makes up for a lack of physicality with a smart stick that takes away the passing lanes.
I still don't really see a huge issue with Scuderi. Like I've said, yes he hasn't been great, but he hasn't been so bad that Engelland is a step up from him.
But I guess we all need something to complain about when everything is going well, huh?