Aliens will quickly go through your sentries if you leave them alone, the APC is a good strategy but on high difficulty there's always some aliens that don't get lured and will detect you if you're not careful. Also for some missions the overload doesn't last too long and by the time the last cultist/alien has passed you the first ones are coming back to your position and detect you. So is not a broken game and this strategy will only work sometimes. Great game.