Nemec is fine. I'd say the pre-draft criticisms of his lack of urgency in puck battles hold true, but otherwise, he's had his ups and downs. When he looks great, he looks like that steady 30 minutes in the playoff top pairing guy. The bad moments are just inexperience, not ability. I don't think his offense is going to be all too special, though.
Whatever disappointment people have is related to NJ's roster situation rather than the player. Development time is over in NJ, it's about the playoffs now and slow cooking all prospects not named Luke Hughes, who has handily outplayed Hamilton. They were never going to keep Casey or Nemec up this year unless they were absolutely flawless. Injuries will happen, and they'll step in.