Yes, I do believe The Edge is underrated. A lot of people don't see how great he is. But he is only underrated by the people who think that the more the better and who can't really do a full search on the man's music. At the same time that he can do a lot with very little (I Will Follow, The Fly, Vertigo), he can make a song really dense and add many things to it (The Electric Co., UTEOTW, Beautiful Day).
It's all about what kind of song he wants to write and what the song needs. The Edge knows that, so you can feel that he is not trying to impress anyone, he's just trying to make good music. At times however I do feel like he retains himself a little to try to prove that point I just talked about, that he doesn't really need to add lot's of stuff to a song to make it better.
But isn't it all about if a song sounds good? I mean if a riff sounds good and original, who cares if he's using the same notes that he used for some other song, as long as what he is creating don't sound too alike it's okay. Every guitar player has a style. It's about feeling a song and making it happen through your instrument!