there are other players who potro has a terrible head to head against, not just the three you mentioned. Don't see what part of that you are having trouble understanding to be honest.
Isn't that embarrassing for them then that he can be so poor but when it really mattered most he was able to maximize his abilities over everybody else like Murray? Fact is Murray couldn't capitalize at the US Open 2009. He was going to face the same Nadal and Federer, but he faltered which gives even more credit to Del Potro. It is completely unfair to judge where Del Potro should be as he went through an injury. Murray, and many other players, don't go through an injury like his. Many players in history don't ever come back yet Del Potro still got back to the top 10. Plus, he has been unlucky with meeting Federer the past few months as Federer will always be a bad match up for him.
He is overrated in a sense that he is over referred as a part of the "real BIG 4" saying he is better and greater than Murray which is not true.....
Fact is he is greater than Murray in the overall scheme of things when placing them in history. No, he isn't the real number 4 because he won't be consistent enough as Murray clearly the more consistent player but clearly Murray didn't have to have a dramatic injury like Del Potro, so it is very unfair to put any type of real expectations from Juan Martin.