Edge can play all the songs frontwards and backwards, his difficult style is in his timing, and the use of the delay/echo. He basically plays major and minor chords, nothing too difficult at all. I think if you "reminded" him of the chords, he would nail any song you asked him to play.
The problems are actually remembering all the chords, changes and style of play, not actually playing them. It's not as easy as it sounds to remember, and I bet there are songs that he hasn't actually played in many moons. But if you said, hey this is "C, Am, Em, F#) whatever, it would be a breeze with a run through, maybe even without.
But that's U2, 3 or 4 chords, sometimes less, basic rock tempo, you could play most of their songs around the same tempo, and Adam has an easier job than Edge most of the time.
So I don't buy the argument of easier music, honestly, I buy that Bono can't remember his own fucking lyrics, so the window of songs to play narrows, so they chose Boy songs to keep the window small for Bono.
And I am not trying to demean the complexity or quality of what they play. I am actually complimenting them, saying I think they could play any of it, it's Bono who can't remember shit.
One example, when asked about the much requested song, Acrobat, a song that has never been played live, Bono said "we'd have to mediatate in a tree to play that one", which I can tell you is horseshit. There are any number of guitar, bass and drums players who could pull off a shitty version of that song on this board, myself included. Edge, Adam and Larry could do it and do it very well, I assure you. The reason they don't play it? Bono.
Bono rehearsed The Fly for this tour, has played it over 100 times live, the song is 14 years old, and he still can't remember the lyrics. Now, there are reasons, I don't remember all of my songs either, so I am notjust chastizing Bono. I am saying this is why the song selection is what it is, among many other things, I am not sure it's because any of it is easier to play than others.
Boy, however is probably the easiest of the stuff overall.