I think that none of the U2 songs are hard to play - insofar as notes, etc are concerned. The Edge tends to be pretty melodic and he is also of the less is more school of thot.
That said, the most difficult thing about playing U2 is getting the sound (effects, EQ) right. Short of using the same effects that Edge uses (and in some cases, the same guitar type) you're going to fine that it's a bit impossible to recreate the sounds on record.
There are some good enuf substitutes: for example, I use an E-Bow for the sustain notes in With Or Without You (in addition to Boss DD-3 Delay, Ibanez TS7 Tubescreamer and Boss CH-1 Chorus) which comes pretty close when played with a Stratocaster.
For Mysterious Ways, I use a Rickenbacker fed thru a Chorus, Boss PH-2 Phaser and Boss OD-2 Overdrive to get that rough crunchy sound in the intro/chorus. For the Zoo TV solo at the end, I add the delay but take out the phaser (using a slide as well).
The trick is playing EXACTLY to what The Edge is doing, because his timing isn't conventional at all. It's mostly done out of "feel" than technicality (e.g. Sunday Bloody Sunday chugga part after the solo; or even the verses for Vertigo).
That's what I find the hardest. But I also agree possibly the solo for the Zoo TV Bullet The Blue Sky is the hardest solo from U2, probably followed by New Year's Day playing BOTH the piano and guitar (like what Edge does live). The rest are pretty easy I find, even the early stuff like Gloria.