He is a bit of a ball-hog in the sense that sometimes he holds on to the ball and doesnt release it to better positioned team-mates, especially when going on one of his trademark runs. Followers will always dispute this notion because everyone has a different threshold of what that "sometimes" should be. For me, yes he does tend to keep the ball sometimes but this is only early in the game sometimes or when the score is stacked in Barca's favour - but if its an important game or the game is on edge, he almost always makes the right decisions which is what makes him invaluable. Which explains his assists.
Overall, I would not call him a ball-hog because that sounds too harsh. Majority of football followers will laugh at you if you tell them Messi is a ball-hog.