But they are, they don't stop, they don't break, their weapons can flay a man, layer by layer. Some of them wear the flesh of their fallen foes, coming at you like something else, others will come through the wall or door, slicing a man in two before they become intangible again. For all of them, something worse is wrong, as if they don't even belong in this world, but one is worse, it's very presence makes your soul wracked in pain, in the body of a cruel simulcra of a man. But for those who draw on the Warp or dwell there, this terror is even worse, where it can rip away parts of them while it is near. Their armor is almost impenetrable to even the powerful melta guns, their flaying guns can weaken or destroy your vehicle with a glancing blow. Worst of all? The silence, the unchanging expressions, the utter hate simmering quietly as they methodically eradicate you and those around you.
Just an idea of how scary they can be. In the Cain books they are really done well. Side note, this is for the original Necrons, not the crappy replacement they got