Seeing that there is a lot of concern or hesitation about McCarthy making Mike Nolan his defensive coordinator, I went back and looked at all of Nolan's stops as a DC paying attention to what I consider the most important stat for the position: points against.
Nolan has been defensive coordinator for seven teams before now. At all but his two one-year stops (New York Jets and Denver Broncos, he had at least one top-10 defense in points against). NOTE: I did not check if any of these teams had a defensive-minded head coach who might have been propping Nolan up.
At his first stop with the New York Giants, he had the No. 1 defense in points against his first year, following by the eight, 16th and 10th. He then went to the Commanders and had the eighth before a bad drop-off to 28th and 29th. (Not sure what happened there.)
In his one year with the Jets, his defense was 13th in points against. Then, he went to the Ravens and had the 19th-ranked defense in that category before being sixth the next two years.
One year with Denver, produced the 12th-ranked defense in that category before he moved on to the Dolphins. There, he started with the 14th-ranked defense in points against before improving it to sixth the next year.
In his last stop as a DC, he had the Falcons fifth in the category his first year before a drop-off similar to the one he had in Washngton (27th the next two years).
So, the good news is that he's had eight top-10 defenses in points against, including the No. 1 defense his first year. The bad news is he's had nine defenses ranked outside the top 10, including four near the bottom of the league.
Again, there can be a lot of reasons for the disparity in his success, but it seems like he's mainly hit and miss. About half the time, we can expect his defenses to clamp down on scoring. And about a fourth of the time, we can expect his defenses to be really porous. (They were middle of the pack five times.)