If he was really bad like somebody here claimed, his stats would stick out like a sore thumb, but they don't. At the end of the day you can credit the defensive system, or you can give credit to Barzal for playing pretty good defense
At the end of the day and I say this as a huge believer in Barzal he's just ok defensively. He needs to do more he has underperformed but thats also partially NYI's fault.
System definitely has an impact though. Jonathan Huberdeau is an example of this going from Brunette to Sutter, His offensive impact has evaporated, and his defensive impact have been pretty good.