Who said that the Flood can't affect Force users? Let's be fair here, the Covenant are just as about as advanced as most of the Star Wars Civilizations, aside from the Force thing. Now, the Covenant are constantly being overrun by the Flood. The Flood are like Necromorphs and an intelligent rival civilization in one. Who is to say that the Star Wars races will fare any better? The only way to stop them is by activate=ing the Halo rings, which would result in a stalemate, not a victory. Also the "Battle Droids" argument is completly invalid. They don't work if the control ship is overrun by a joint Spantan/Flood/Scarab team.