Perhaps it's because the band sounds like it uses the same general riff over and over and over. I'm sure he's talented but the band hasn't used it properly. When the first single from Black Ice came on the radio, I knew it was AC/DC before Brian Johnson's voice even began. If it's that easy to pick up, then perhaps it's time for him to expand his style.
Edit - Don't get me wrong, I like the band but I still wouldn't rate him as high as a lot of other guitarists. For me, being the same leads to boredom and I like when artists take chances - it shows they have b*lls. It doesn't always work out true, but I'd still take that than the same ol riff every album. Oh, and I have seen AC/DC - three times even - and it still doesn't change my mind.