I think everyone knows Nalbandian's backhand and overall touch (including at the net) are sublime, don't think his volleys are underrated.
A fine example of an underrated shot is Tsonga's serve, which is clearly the best in the top 10 now that Isner is #11. It's basically the shot that has allowed him to have such a good career despite having the worst backhand and return in the top 10 (again, thanks to Isner being out of the top 10 at the moment).
Ehhh I think when people think of Nalbandian they automatically think of his backhand (which is amazing) But he doesn't have a bad drive volley either, it's not all drop volleys
And I also think everyone knows Tsonga's serve is dominant. There was a whole thread dedicated to him holding (was it 80+?) times. So no doubt Tsonga's serve is recognized.