It's ok to go to a show if you are not an expert on the material or even if you don't know the band at all. I was once taken to a show of a band I had never heard of, turned out to be The Alarm and one of my favorite shows ever. The first time I saw New Order, they were terrible, I left. Big Audio Dynamite was a huge disappointment too, The Replacements were too drunk to stand up. People do have bad concert experiences and it's ok to say so. Also, you are allowed to prefer U2 over JD. Everyone loves Joshua Tree whether they admit it or not, even if they are like me and see it as the sell out point for U2. Bono is a huge JD fan.