He has 11 slams at the age of 26 and the greatest record on clay ever. How is that underachieving? He has worked hard to maximise his talents and if anything, given the position his career was in at the end of 2005, with the real possibility of a congenital foot problem sidelining him from professional tennis completely, he's achieved a remarkable amount.
As for those whining about surface speed, their favourites have also won on the same surfaces and there's no knowing how they would have fared if courts were much faster too. You'd think every tournament played like the French Open when it clearly doesn't.
Nadal has nothing to regret. I'd say Djokovic is the underachiever if anything - he basically wasted a period from his first slam in 2008 to his second in 2011. Only really started contending again for slams by US Open 2010 - had a lot of odd losses and slumped for most of 2010. Nadal, even in his worst years, has won a slam and a masters. With Murray, I think he's always tried hard and come up against better players so can't put down his lack of early slam success due to underachievement.