I actually think being Pau's brother hurt him. I think in a lot of ways he was being compared to him, and this led to some false reports about his skills. Before the draft he was being billed as an unskilled offensive player, who was known for his defense and rebounding. And I think it was more along the lines of "he is not skilled like Pau", but it is pretty obvious that he did have some decent skills, and was still developing.
Combine that with the fact that he has a similar build to Brian Scalabrine, which can lead people who have not seen him a lot to think he is not very athletic, and he was destined to slide. Being in Europe made it a little tougher for scouts and GMs to judge his athleticism, because he was playing with and against other guys who were not NBA level athletes.
So he ends up being branded as a raw offensive player whose strength is defense and on the boards, however, he does not seem like a great athlete, which may prevent him from being a good NBA defender.
Clearly, a lot of guys missed the boat on him.