It's always hard to speak of "overrated/underrated" because it depends in which one's eyes, and also I'm not enough of a technical expert, but I will try speaking only of top-top-players :
I think Fed's forehand is overrated comparing to Nadal's ... I mean now
, because in the past his forehand was even better than Nadal's has ever been.
Yesterday, NYCtennisfan was speaking about how Fed's forehand had changed and showing some clips. He has made it change because of his movement declining on that side, and it's another one of Fed's great evolutions he has made in his game, but it's no longer an equivalent of Nadal's forehand. Fed's serve is also often overrated : especially during last Wimbledon, I read that he had defeated Djokovic and Murray thanks to his serve whereas he had the same number of unreturned serves as Djokovic and nearly half of Murray's unreturned serves. But it's also related to his back problems probably.
As for Nadal, I think his footwork and his serve are underrated but his backhand and return have been overrated in many articles I read. I remember the bullshit that his backhand was the best of the game because he was ambidextrous then had 2 forehands
Nadal's mental is also great but overrated imo but that's like every myth : it goes too far.
As for Djokovic, I think his return is a little bit overrated at the moment. It's very good but it's not like Agassi's and Connors's quality shot in the past considering the context of their time. On the opposite, his mental is still underrated imo. His touch is also underrated imo.
Murray's backhand may be overrated as well, doesn't make many winners with that and is less good for the passing-shot than with his forehand, but his return is still underrated imo.