I think SS Necrons are balanced well, but where they fall apart entirely is Warriors. I'd like to explain why I think that is. I'll leave vehicles entirely out of the equation, as I want to focus on infantry options, which are the primary force of the Necron military. Look at the Necrons' infantry options:
*Pariahs - fast-moving melee secondary commander
*Necron Lord - fast-moving primary commander
*Wraiths - fast-moving melee unit
*Flayed Ones - very slow tank/melee units
Let's stop there for a moment. I've already named off over half of the Necrons' infantry options, and they're all melee. Half of those options are Commander units.
The other half have serious limitations:
*Wraiths are not an option in T4 games. In early tiers they are relegated to being used against Tau and Eldar. They cannot reinforce, and are sometimes lost from reference keys when they are KDed. They are definitely lost from reference when they resurrect. They cost 2 pop, despite being a single unit, and have little to no use beyond T2. The bottom line with Wraiths is that they are not a fast Flayed One squad. They cannot take hits, and despite being able to approach while invulnerable, they fall very, very quickly. As a result, they are inconsequential in T4.
*Flayed Ones are too slow to catch anyone that is even half-heartedly microing a squad. This results in players using them for base spam, or as "linebackers" for ranged squads. They have no squad cap, and they have a lot of health, so this only reinforces the use of this tactic.
Now let's consider the Commanders:
*Pariahs are a wonderful unit. They are difficult to take out without proper firepower, and have an interesting ability in lowering maximum health. However, they are truly a disruption squad, not one made for damage. They tend to knock back enemies more than damage or kill them. They do, however, work well against relic units or larger units like Daemon Princes. This does tie them all up into one target, however.
*The Necron Lord is the ONLY infantry unit the Necrons have that actually has an active ability, aside from the standard teleport function. The three special abilities a Necron Lord actually gets are often very easy to counter, either working or not working at all, or cannot be used actively. Some are debatedly necessary just to keep your Necron Lord alive throughout the game, such as Phylactery or Phase Shifter. So, a Necron's arsenal of special moves is usually limited to a SINGLE ability across EVERY unit on the field, like Solar Pulse.
As you can see, Necrons' melee options are fairly static. Flayed Ones, by necessity, make up the bulk of a Necrons' late-game melee force. These units do not have any special abilities, aside from the limited use abilities provided by the Necron Lord. Necron melee units are typically disruption units. They are not made for fast approach and fast killing power.
Now, let's look at the infantry ranged options:
*Immortals - fast-moving, vehicle-centric units
*Warriors - slow-moving, infantry-centric units
You will notice there are only two options. This is where Necrons begin to break down as a T4 force. As melee units are not the primary damage dealers, and are made primarily to tank or disrupt, the task of dealing damage is left to the ranged infantry. What are their limitations?
*Immortals' primary problem is their very low health. They die amazingly fast, and have a very low number of units per squad, meaning that once they begin to take damage, it's nearly impossible to save them through reinforcement. This is not unfair, as they have both wonderful vehicle and building damage, and enough infantry damage to almost mete out the same damage as a fully reinforced Warrior squad does. Their health alone is what keeps them from being used as a Necron's primary ranged infantry option.
*Warriors are very slow, have a short range, deal an inadequate amount of damage, take up 3 population, are difficult to build quickly, deal next to nothing in damage against buildings and vehicles, and also have a very, very low amount of health in comparison to their many other shortcomings. Warriors are, without a doubt, the game-breaking problem for Necrons.
Let's look at the tabletop version of Necrons for a moment. What's most important for them? Formation. (Remember that.) Warriors make up the bulk of Necron forces on the tabletop for various reasons. They typically hold close to the Necron Lord and the Monolith and make use of the WBB (resurrection) rule to revive. A common complaint with Necrons is that they have basically one or two ways to play them. They're boring. Ignoring that, if it works, it works. DoW is meant to carry this over. Now, can you play this way in DoW? No.
A typical formation a TT player might use in DoW would probably look something like this: A Restored Monolith, with four squads of Necron Warriors (12 pop) placed directly in front of it. Amongst their ranks is a Necron Lord with Resurrection Orb, possibly Phase Shifter or Solar Pulse, perhaps Phylactery. All of these units will likely be on a Ranged stance (F2) to keep the Necron Lord's Resurrection Orb ability passively affecting them all, and enhancing the chances for the WBB rule to occur. To the sides of the Monolith will, likely, be two squads of Immortals (4 pop, 16 total). Although we are ignoring vehicles for the most part, there would also be Tomb Spyders involved, picking up bodies and adding back Warior squads when they finally do fall. Pariahs are not a popular choice in TT play due to their awkward pacing. I do not believe our theoretical TT player would pick them up. Instead, the rest of his force would likely be either additional Warriors or Flayed Ones. (Wraiths also have issues in TT.) Let's say he chooses to include 1 Flayed One squad (3 pop, 19 total). That takes us to 19/20. Let's go ahead and say that he's collected some bodies and now has 2 more (3 total) Flayed One squads, for a total of 25/20 population. He intends to use these units to deep strike towards isolated targets, damage or kill them, and then teleport back while the rest of the formation successfully holds. Would this formation, which would be quite typical and moderately successful in TT play work in DoW? Of course not.
Why would it not work? Warriors. The Warriors would die so quickly that nothing else would be able to do its job. The theoretical notion of how Necrons should work is that they should revive enough and take enough damage for Tomb Spyders and the Necron Lord to continue to replenish their ranks. Even more than that, they are meant to TANK damage, while barely outputting any of their own, and slowly wear down an enemy force. In DoW, a force like this would easily be obliterated in seconds with a mass of bolter fire. A Warrior, operating at a mere 850 health, would revive at 30% of that health (255 health). That 255 health will last the unit long enough to complete its revive animation, at which point it will immediately die again. The Warrior force would crumble in seconds, and the entire formation would fall apart relatively quickly.
TT is obviously not DoW. But how did the fluff and TT translate to DoW? Obviously, melee units are not meant to be the bulk of a Necron force. They are slow and disruptive, not long-ranged and high-damage. Obviously. With only two infantry options, one must be the primary force. Immortals cannot be, for reasons explained. This leaves Warriors. Warriors are meant to be the primary force upon which the entire army (and FORMATION) of the Necron army forms. Necrons are meant to hold that formation and to work as a large force, rather than splitting up. Thus, the success of an army depends on the ability of a Warrior squad to successfully maintain a formation.
Without a successful formation, Flayed Ones, Pariahs and the Necron Lord have to cover even more area. This extends the line of attack and spreads apart individual units, making them easy to lure off and surround. When this is not the case, melee units become clumped together, and fumble around buildings and each other to reach their targets. This becomes wasted resources, as only a handful of the units are actually doing anything relevant. In any case, you do not want a very large force of melee units - especially not very slow ones that are not made for taking targets down quickly.
The inadequacies of Warriors leave Immortals to take the role of primary damage dealers. Obviously, this does not work well. They die so easily that they are forced to move inside of a melee formation (like a quarterback in the pocket of his linebackers) to avoid melee attackers. This leaves them very vulnerable to ranged attacks. They can hold far away from combat, but this leaves them very vulnerable to attack, and excludes them from what they should be doing: removing important vehicular threats, and buildings. They are a good unit, but they have no business doing the Warriors' job for them. It creates an unstable formation, which means enemies are not controlled and your own force is erratic and unbalanced.
Inevitably, the issue with Necrons is that for how they exist in DoW, Warriors need to be able to tank damage, while having a very short range and inadequate firepower. A Necron player should not be microing his units' abilities so much as attempting to position them properly, for maximum affect of passive abilities and holding a formation. The only thing SS 1.2 does NOT get right in this regard is the Warriors' ability to tank damage. In DC 1.2, this was done - more or less - correctly, but due to what I chalk up to as politics, vocal players that were as mad at DoW Necrons as TT players are at TT Necrons for their ability to withstand huge amounts of damage and slowly wear people down, eventually resulted in what is debatedly the worst race in SS 1.2.
Necrons do not necessarily need more firepower, more range, a change in Wraiths or for Tomb Spyder scarabs to be returned to a melee-capable unit. They need Warriors to be able to do what they're meant to do: withstand large amounts of damage through having a decent amount of HP to go with their ability to resurrect, ignore terrain modifiers and be replenished by Tomb Spyders on the field. It is my opinion, as a result, that Necrons simply need another Warrior health upgrade, in addition to a slight modification to their revive ability (which also includes the Resurrection Orb modifier).
Having more resilient Warriors will result in better formations, which will result in less dependence on Flayed One spam (which is not fun for either party) and less chance of an entire Necron military (which is very hard to produce to begin with) being completely wiped out in less than 30 seconds.