This is rather subjective so I'll make a few names compared to my expectations.
Underachievers: Moreno Moser, Robert Gesink, Roman Kreuziger, Carlos Betancur, Peter Sagan, Jan Ullrich, José Rujano, Thomas Dekker, Marcel Kittel, Andy Schleck.
Overachievers: Fabio Aru, Esteban Chaves, Ryder Hesjedal, every rider who won a Monument in 2011 bar PhilGil, Paolo Savoldelli, Simon Gerrans, Elia Viviani, Mark Cavendish.
I did consider Nibali an overachiever up to 2013. His Vuelta win looked liked a mix of mental strength and lack of top riders, while his GT podiums were mainly due to team strength. I'm pretty sure back in 2010 I wrote on a forum something like: "Vincenzo Nibali won't win much in his career, but he will rarely lose a race he can win". And I was wrong.
Since 2013 he has been top4 in every GT he has targeted and finished. He has 11 GT podiums overall so it really comes as no surprise he won 4 of them.
He only trails Gilbert and Valverde for Monument podiums among active riders. There's volume beneath his big wins, so there's no way I can consider him an overachiever. He's a top class rider.