With his skillset he should/could have won practically every important race on the calendar except for the northern classics. Plus many more stages everywhere.Don't really see Valverde as a big underachiever, outside him never winning Il Lombardia. Incredible longevity and consistency, but not the climbing peak where I'd say "multi GT winner". Maybe he could've won another Vuelta, but it's not that I consider him an underachiever because of it. Also wouldn't see him as an overachiever.
The man has been one of the most conservative/non attacking rider I have seen. And thats because of his lack of confidence.
If he would have had a better team could have help.