You're never going to make everybody happy with your choice of opening acts. Particularly when it comes to U2 because they have this knack of drawing fans from a full spectrum of musical genres.
Generally I think U2 go with bands that they enjoy where possible.
You also have to keep in mind that supporting acts do have tours of their own and aren't always available. For example, I'd love to see The Arcade Fire opening the Toronto shows, but they're going to be in California at the time, so that ain't really possible.
So let's keep some perspective. Trying to make everybody happy usually ends up making everone miserable.