I just don't buy the "Melo isn't a winner" label.
The one time Melo was surrounded by proper complementary players and a real NBA coach, he gave the eventual champion Lakers their toughest fight in the WCF.
Put Melo on a team with Rondo and a solid defensive big, and I definitely think you could win with Melo. I don't know whether or not Boston would be able to put the necessary pieces around Melo and Rondo, however.