As far as him being overrated, I have a few reasons.
The biggest reason for me personally is that his style of wrestling, at least in his recent high profile matches, just isn't my style. I don't mind kicking out of a finisher on a big stage, it does build drama. That said, how many Rainmakers did he kick out of over the course of his trilogy with Okada? Too many, I know that much. It isn't a Omega specific issue, at No Mercy Roman(one of my personal favorite WWE guys) kicked out of I think it was four AAs, including a super AA. That just kills matches for me.
He also no sells a lot. I'm not going to say that a complete lack of selling is bad either, sometimes the whole fighting spirit or second wind thing makes sense, but when you are twenty, let alone thirty or forty minutes into a match like he often is, stay down for a few seconds before you hit your next big move. Don't just fly around the ring the entire match. Sell leg damage. Limp.
Speaking of, a lot of his matches are just big move after big move after big move. Again, that can be fun sometimes, but I feel like every match of his I've ever watched is just him using his athletisicm and signature moves to pop the crowd. By his own admission he couldn't even wrestle a coherent match until he was signed to WWE in 2006. I feel bad that they assigned him to Deep South Wrestling, probably the worst development territory ever though.
Then most of his fans are insufferable. He is good, I'll never deny that, but lets not act like he is the reincarnation of Jesus Christ as a pro wrestler like Dave Meltzer does. I'm 27, I've been watching wrestling since I can remember, probably a good twenty two years. I'm not KB, but I've seen a lot of great performers, and while Kenny is good and ocassionally great in my eyes, he isn't the best, isn't close to it. I hate that Meltzer and others put him on a pedistal just because he wrestles sixty minute draws in New Japan.
Oh, and the Bullet Club annoys me, but that isn't just a Kenny specific issue.
All that said, his Hadouken is one of my favorite spots in wrestling.