Quantum Shielding is probably making the biggest splash with the new Necron book, and it looks to be an easy way to put a lot of armor on the table and give the enemy problems. However, it's difficult to get a grasp on just how tough it actually makes you, because it ends up being a very complex situation. Having been thinking about it a lot myself recently, I'm gonna try and put down a bunch of my thoughts on the subject and try and sort things out a little.
Let's hit the obvious first: AV13 is tough. Really, really tough. Having it on both the front and sides (not to mention Rear AV11, which is also unusual), Necron vehicles with Quantum Shielding are incredibly resilient so long as the effect lasts. On the other hand, they are all also open-topped, meaning that hits that go through always have a chance of wrecking them and AP1 weapons are very punishing.
Obviously, Meltaguns and Railguns will devastate such a vehicle if they get to do their thing- but this is to be expected, since these guns are specifically designed to kill even the heaviest of tanks. On the other hand, anyone who's rolled on the vehicle damage chart knows just how tough it can be to bring down a tank, even a wimpy one, as sometimes you just rolls strings of low numbers. Open-topped will mitigate this somewhat, but it's still a distinct possibility.
Another common argument is that losing Quantum Shielding after the first penetration is irrelevant because "you'd be dead then anyways." NO. Absolutely wrong. Even with a Meltagun, a successful penetration is only 66% likely to actually kill you- again, this is the strength of the vehicle damage table in action. Even under the absolute worst circumstances possible, you get an imaginary 5++ save to not die when you otherwise should; against most other weapons, you are still looking at 50% chance of living through the hit. It will be quite common for your vehicles to continue flying around sans Shielding, so don't just assume penetrating hit = dead.
So just how vulnerable are you? Let's run some numbers.
Mathmatical AntiTelharsic Harfatum Septonum
Vs S7 (per shot)(I am ignoring Shaken results here because the Ghost Ark is largely immune to them and they aren't really part of the discussion. Living Metal has already been applied to the Stunned results for the Ark. Also, all shots are made assuming BS4, for simplicity's sake.)
-Rhino: .07 dead, .11 damaged, .06 stunned
-Necrons: .02 dead, .04 damaged, .01 stunned
-Post-Shields: .13 dead, .12 damaged, .03 stunned
So, unsurprisingly, the Necron vehicles are all but immune to S7 guns, requiring several score shots to achieve even a single damage result. Rhinos, while certainly not dying in droves to them, CAN be killed or hurt (Immobilized results especially hurt them) and, unlike the Necron tanks, are not immune to suppression effects. But, as a rough measure, Quantum Shielding leaves you about three times as resilient to non-suppression shots, which is very good considering Necron transports run about three times the price of Rhinos and come with more guns, etc.
S8Strength 8 weapons are another important measure because they can be found in abundance in today's environment, mainly thanks to Space Wolves and Dark Eldar. I'm ignoring Lance here because it's not 100% clear how the two will interact (although I suspect it will leave you with AV12) and because it just muddies things up even more.
-Rhino: .11 kills, .15 damaged, .08 stunned
-Necrons: .08 kills, .08 damaged, .02 stunned
In any case, we can see that Necrons are still quite tough, although by a much smaller margin; they are about 40% harder to kill and twice as hard to hurt, and the numbers on suppressing them are still so low as to be negligible. You're still looking at a dozen or more shots to wreck a single vehicle, meaning that people will have to dedicate tons of firepower to you to actually do anything.
S8 AP1Ah, Meltaguns. Even here we can see a distinct superiority on the Necron side (60-100% more survivable, extremely unlikely to be suppressed) as AP1 tips the balance against the Rhino and makes Open-Topped less of a disadvantage. Without double-pen dice Meltaguns will still struggle to kill Necrons simply to due high AVs. S8 is still just S8, even when you have other fancy stuff helping out.
-Rhino: .18 kills, .15 damaged, .08 stunned
-Necrons: .11 kills, .08 damaged, .01 stunned
When you factor in double-pen, Rhinos and Necrons are basically the same; I won't list everything out here, but the slight bump to AV is negated by open-topped.
S9S9 is where we start to reach parity; Necrons are still more likely to avoid losing a gun or getting stuck, but Open-Topped takes its toll and brings things down to almost-even with the Rhino. Note, however, that you are still avoiding any meaningful chance of being suppressed here.
Rhino: .15 kills, .19 damaged, .10 stunned
Necrons: .13 kills, .11 damaged, .03 stunned
What It Means
Against the most common weapons in the game (Multilasers, Scatter Lasers, Autocannons, Missile Launchers, Lances, Meltaguns) Necrons perform quite well. It's only the rare "heavy" guns like Lascannons, Railguns, and artillery that their high price tag will fail to yield similar levels of survivability bonuses.
Just as importantly, Living Metal means that traditional anti-armor tactics will simply fail against Necrons, much as they do Grey Knights. Though Quantum Shielding tends to get the most press, its cousin is omnipresent (even on the non-Quantum vehicles) and will make shutting most transports down hellishly difficult when combined with the high armor values.
All in all, this gives Necron vehicles a very similar flavor to their infantry- very resilient, if somewhat slow in many cases, and able to shake off all but the most determined of firepower. Just as focused fire will be needed to drop whole squads of infantry to prevent RP rolls, knocking down the shields and then destroying Necron vehicles will require some concentrated directing of firepower, which in turn necessitates good target priority selection.
Other considerations
So what happens once you lose your Shielding? That's actually rather simple: just use the Rhino numbers from above and slightly-less-than-double the number of killing hits. (The actuality is a little more complex with that, but it suffices.) Even post-Shielding, Necron vehicles are not terribly easy to destroy by virtue of being vehicles. Killing vehicles is hard, y'all.
Cover will, in general, be harder for Necron vehicles to achieve than Rhino chassis, not only because of size and shape but also because of being on the skimmer base. You can layer your vehicles as usual to get it, but that still leaves a front row unprotected because you don't have a Smoke Launcher equivalent.
Although the Monolith is the only vehicle with an actual trait to make it slow, many Necron vehicles find themselves wanting to sit still or move slowly in order to fire to full effect; only the Night Scythe and Doom Scythe are really going to have the kind of mobility normally associated with skimmers. (Command Barges technically do as well, but they will generally be jetting 24" in order to sweep something and get a cover save, and thus function rather differently.)
