They are the most useful in nearly all situation and have an impact on gameplay. These are the best upgrades of the list to choose from available to the Necrons.
But please take Pyramidal Reconstruction, I can’t overstate this enough. I believe that the ability to do mass repairs on your ships and keep them in the fight longer is worth more than the extra mobility and damage the other skills offer. Scarab Swarm I think is the second best skill by far on this list, though this is preferential. The closest comparison to this is thinking of an Aeldari ship that has had its engines destroyed… Those are dead Eldar. Still dangerous but they will not achieve anything. The Necron fleet relies on this ability heavily, without it all their ships are paper weight. While Necron ships do repair light critical effects over time, if your engines are completely destroyed you will lose access to your teleport ability. Among these four, Pyramidal Reconstruction is a must have in every fleet. Out of all the skills, these are the four to choose from. If there is a skill or upgrade that you like that doesn’t make it to this list, I encourage you to try it and find what works best for you.
I will outline what I think are the best and most important in the following. Unlike some other factions there are far fewer options when it comes to what is good with the Necrons in the skills and upgrades department.
