Pretty bad, really.
This article from Panthers.com gives an idea as to how bad. It also gives a solid feel for how much better the last two years' work on the team website has been since the Brett Borden days, but that's a separate topic.
So, quickly, here are the highlights:
*Carolina had not had a blocked punt, kickoff return, or punt return in the five years Crossman was special teams coach.
*Carolina gave up five special teams touchdowns in that same span, and had five punts blocked in that span.
*Carolina ranked 32nd in kickoff return numbers over the same span.
Crossman was a hard-working, dedicated coach who put tons of time into his craft. But it doesn't appear he's any good at it.
With that said, he also just got hired in Detroit.