Carter's NFL career was downright tragic. He was perfectly healthy all through college, but took one bad step at the crappy Cincinnati astroturf, and it was all downhill from there.
As for 1990s PSU tailbacks, the real question is how Curtis Enis ended up a bust.
Enis was even better than Carter. The 1996-97 OLs were okay, but nowhere near as good as 1994-95, which were just flat out incredible. Kyle Brady (1994) is also the greatest blocking tight end ever - Carter had him, but Enis didn't. And Mike McQueary was an atrocious passer throwing to mostly meh receivers. Enis carried that team. In college, he was every bit as good as, say, Eddie George. Probably better. I'm still perplexed how he didn't do much of anything in the pros.