Necrons are "extremely arrogant aliens that feel like they cannot die," so their weakness in Dawn of War 4 is an obvious one: greed

Dawn of War 4 's Necrons are the hard nuts of Warhammer 40,000. Each one of their soldiers is a >soul trapped in a living metal body, practically immortal thanks to the faction's reanimation technology. Like a clothes moth infestation, once they've made their home on a planet, they're virtually impossible to get rid of. They lend themselves to wars of attrition, constantly expanding their borders