>>8815835Game wise, they are typically slow and tend to have fewer units. They can't always come back to life, there is a chance of it failing. They also use to be very strong but got nerfed to the ground then brought back up recently.
Lore wise? They are a very fractured race with each dynasty having its own motives and goals, this means they almost never will unite as one large army to wipe out the others. When they show up, it's usually going to be small numbers that can potentially be overwhelmed, however their tech superiority makes even that advantage a difficult one to make use of. They also tend to be more defensive in nature, protecting their tomb worlds instead of going out and destroying everything, some do but usually they are sticking on their tomb worlds. And finally a large number of confrontations occur from interaction with accidently awakening a Necron tomb in stasis, it takes a while for the tomb worlds to reach full operational status, this does help out giving an edge in taking them out before the entire might of the Necron on that planet can fall upon the intruders.
But it's still super risky even with that, they are very much a powerful force in the universe and feared by many.