He was awful. The Devils were outscored with him on the ice 5v4, and they gave up 2.5 xG/60 which is obviously atrocious. Everyone on PP2's numbers were better without him (and likely better without him and Hamilton, too). Among NHL defensemen who played 50 minutes on the power play last year, Nemec had the worst on-ice shots ratio and by far the worst xG.
I don't doubt that at some point, Nemec will be a fine PP guy, even if he's not asked to play there that often. I'm skeptical he is any good now.
$6M is too much to pay for that, you can replace that for less plus the buyout, and again, if he is that now, there's no telling that he would be that next year, either. But this is all a long way off.