2BlackJack1 wrote:
I mean, getting through an entire tomb without setting off a single failsafe and getting yourself a one way ticket to a bloody death is a very difficult task. And then you have to take control of the necrons, who might just self detonate as another failsafe, but in theory it could be done.

 Sigvatr wrote:
Necrons are soulless automatons, but they do have a lot of subroutines. Entering a Tomb World triggers the defense protocols of anything in it, including Necrons and all defense systems the Tomb World has. There's more defense than just the regular army

On the other hand, that's one of the two weaknesses the Necrons have - Tomb Worlds. If you destroy a Tomb World, the entire "population" is gone. Forever. Destroyed. Every Necron of said Tomb World in the field that suffers critical damage will destroy itself and can no longer be repaired. At the same time, though, I guess that the C'tan would interfere if the Necron enemies (read: everyone but Tyranids) decided to large a full scale battle...which will not happen as the IoM vastly underestimates the Necron army and the threat it poses, it even purposefully spreads propaganda displaying Necrons in a much weaker light (i.e.: Codex: Necrons 5th).

The other weakness, of course, is The Warp as even the C'tan cannot manipulate or interfere with it. On the other hand, Necrons are immune to all non-material / non-kinetic Warp powers. Without souls, they cannot be manipulated in any way.