Evans and Schleck both should have won more than one grand tour. Schleck was more of a natural talent but if he had Evans drive he would have won multiple grand tours. But Evans also won a world road title which is a very difficult race to win which makes Oscar Freire's record quite amazing but he also had the talent, and sprint on the right course.
Robbie McEwen probably underachieved by never winning a monument or a world road title.
Evans was solid, consistent GT rider but ultimately I don't think he had the climbing wheels to consider him an underachiever for winning one Tour and getting podiums in the others. Both the Tours he got second in he had very favorable routes, in 2007 Contador was mainly occupied with Rasmussen and in 2008 he lost a truck of time on 3 MTF to lose the race. Then he needed a huge stroke of luck to win in 2011 anyway.
Schleck yeah, should have won more than one based on his climbing alone.
I'd rate Bardet as a bit of an underachiever.
I'd also say I'd expect Cadel Evans and Valverde to have won 2 or 3 more GTs (especially Evans).
Perhaps Samuel Sanchez was also an underachiever, as I'd think he could win 1 or 2 Vueltas.
Vino could have won more GTs also.
Van den Broeck also. Guess that injuries screwed him.
Same for Esteban Chaves.
Daniel Moreno (perhaps I'm being kind of irrational here, but I considered him to be able to fight for podiums here and there at GTs).
Nieve.
Talansky.
Kashechkin was an overachiever (never really liked him along with almost all of the Vino entourage).
Menchov imo was also an overachiever.
Juan Jose Cobo also?
Aru.
Meintjes.
Rui Costa.
Samu could've won a Vuelta, but that's about it. Might have been more interesting for 1 day races. OG makes up for not winning a GT IMO.
Vino I wouldn't know. OG and 2 LBLs already, don't think he got close once apart from that Vuelta. Think he's an overachiever.
Van den Broeck, perhaps an underachiever but no GT winner at all.
Dani Moreno finished 9 consecutive times in the top 12 in La Vuelta, with a lone 5th place as a top 5. Kind of guy where if he's gonna win big, it's more likely to be a luckyish LBL or Lombardia. Maybe what if he hand't been a domestique for Purito, but overall really solid rider.
Chaves is a good shout based on 2016, but I think he overperformed that year and has been underperforming since. Don't think he'd underperform by not winning a GT, cause he's one of the worst TTers of the lot and he doesn't exactly fly away from everyone to make up for it. That 2016 Giro was the ultimate shithouse win from Nibali, but ultimately he just dropped Chavito badly.
Nieve, hasn't won a Tour stage and that's the only underachievement I see. Great climber wish he'd gotten more chances as a stage hunter.
Talansky, don't really know
Agree with the overachievers, though Cobo did have the mental health issues.