Only nightmares with Karlsson anymore are all the games missed and the huge contract, but yeah that thing in Ottawa just really went sideways.
He's still had 85 points in 109 games for the Sharks. That's an undeniably elite talent from the blueline.
He broke his thumb this year. Could happen to anyone. Not sure I'd call that a big career injury concern.
EDIT:
Looked up defender production over the last 2 seasons. Here's the list of guys who have played the last 2 years and have more production than Karlsson.
Carlson
Burns
Krug
Hedman
Josi
That's it. San Jose has been underwhelming, but I'd say Karlsson is producing about a top 5 level is about what you'd expect. You'll note his teammate is #2 on that list. It's not surprising when you have two elite options, they both won't produce at the same level as if they were the only option. They are the only teammates to appear anywhere in the top 20. Since Karlsson's injury, Burns' production has increased this season due to getting all the minutes.
So yes, it's a big boy contract, but he's still in elite company. We're not talking about the ghost of PK Subban here. The Senators are far worse off without Erik Karlsson right now, contract and all.