well the general problems are thus fold for necrons:
they are not a CC type of army, CC got much better in 5th
Necrons relied on glancing hits to take out tanks most times in 4th, Glancing hits took a hit.
Necrons have always been an "easy to play, hard to master" kind of army, where they are, by far, easy to figure out, but once you start facing skilled players they become very unreliable with their lack of any real choice, especially since they, by far, have the least choices out of all the armies if im not mistaken.
1 Valid non SC HQ
3 Elite choices, only one semi Valid
only 1 troop choice
3 fast attack choices, all with their uses
3 heavy support choices, all have their uses
so if you count the 2 SC aswell, you have 11 useful unit types. 14 unit types overall.
plus the fact that theres 0 customization to your units outside of the single non SC HQ.