Well, not necessarily. Suppose somebody from the Hyugga clan does fight somebody from the Uchiha clan, they're aren't guranteed to lose. What if the Hyugga fighter(s) don't use the byakkugan knowing that the sharingan could take serious, deadly advantage of that? And in a fight between Neji and Sasuke, for example, i Believe Neji would just knock the living shit outta Sasuke. Itachi and Neji would be up to anyone else's guess.