While it is arguable that Necron warriors are some of the best troops choice at their cost, having better armor than even Space Marines (3+ and 4+ to repair), plus the ability to destroy ANY armor value (even void shields on titans!), and that Destroyers are a superb Fast Attack choice, Necron Lords have some of the sweetest abilities an HQ can have, and our Monolith is by far the best tank in the game, there are a number of dead zones in the Necron codex.
Lets start with wraiths.
Now, some may argue (and they would not necessarily be wrong) that wraiths are a great and powerful force to be reckoned with. It is true that they have perhaps the most speed and versatility of any unit of their type, and arguably in the game, not to mention a 3+ invulnerable saving throw, counting as Necrons, and a We'll Be Back roll... but I believe wraiths are not worth including in a Necron list, due to some restrictions with their deployment, and the fact that they deter from the army, which is essentially a shooting army, with strange powers and tricks.
Backtracking a little, lets look at the elite section of the Necron codex.
Here we see Pariahs, Flayed Ones, and Immortals. Call it codex-writing barf, but we have a distinctive two types of units in our elite choices that are not capable of supporting themselves, or the rest of the army. Let's face it, a Necron alone isn't scary enough to most enemies, and a decent space marine would probably be able to kill a single Necron warrior. The entire purpose of the Necron army is its vast wall of souless, silent warriors staring into the abyss with eyes full of malice. Poetic no? We'll the flayed ones (while interesting in the story) are just not part of that conformed, organized robotic menace that is the Necron army. Wraiths, though being good units in general, are doubly so. Even our Pariahs are unfortunately sub-par, despite being (on paper) absolutely amazing!I believe (though I realize some do not agree) that a Necron army should never take these units, because they do not support the whole of the army. Wraiths are too expensive, and you can only take 3 PER FA slot! Flayed ones while being able to make it close to the opponent with deep strike and infiltrate do not do anything to support the rest of the army, and because they are constantly out on their own, when they die they often are not allowed their WBB rolls, which makes their cost rather wasteful. Pariahs could be spared from this judgement if only they were "Necrons", and knew what they wanted to be. They want to be long range, they want to be in hand-to-hand combat... at almost 40 points per model, and not counting towards phase out I think they are a terrible choice in comparison to the lower-costing Immortals.
In essence we have a half-working codex, in an already harsh 5th edition environment. I never give up on my ridiculous crusade to continue playing these metals bastards though, because I find the fun in the theme. You can play all the armies in the game until you find the one that just wins you the most games, but it really comes down to which one you think is the coolest.