Only pick naga if you think your team can fight 4v5. They don't need to win, they need to break even. Don't pick naga against too many heroes that can easily deal with illusions: OD, Lion, Sven, Tiny, Earthshaker, ... One of them is OK (except maybe sven or tiny, you can't manfight them), but three is too many. Pick naga if the enemy team is not very good at pushing.