Some notes:
JANEWAY: They have an extraordinary immune response. Anything that penetrates the cell membrane, chemical, biological, technological, it's all instantly destroyed. That's why the Borg can't assimilate them.
EMH: Resistance in this case is far from futile. Nevertheless, I believe Borg technology holds the key to saving Mister Kim.
JANEWAY: How so?
EMH: I hope to unleash an army of modified Borg nanoprobes into his bloodstream, designed to target and eradicate the infection. As you know, I've been analysing the nanoprobes. They're efficient little assimilators, one can't help but admire the workmanship, but they're no match for the alien cells. So I successfully dissected a nanoprobe and managed to access its re-coding mechanism. I reprogrammed the probe to emit the same electrochemical signatures as the alien cells. That way, the probe can do its work without being detected. Observe. The alien cells are completely oblivious to probe until it's too late. Unfortunately, I've only created a few prototypes. I'll need several days to modify enough nanoprobes to cure Ensign Kim.
I see this was interpreted by Nattuo as indicating the Borg couldn't figure out how to assimilate 8472. However, there's a very real difference in what's going on in this case vs an assimilation. Here, the object is remaining undetected long enough to destroy the cells in question; in an assimilation attempt, the nanoprobe must remain intact long enough to
significantly alter the cell.
Unfortunately, there's very little resembling
actual relevant evidence being produced with respect to WH40k in the SBC thread, although this is normal. It's generally just hyperbole.
So I'll make a list of what I see the relevant questions we need to address here to address the question of Borg assimilating Necrons.
Step 1: Acquire Necron to assimilate.
This is not necessarily trivial. The Necrons make use of teleportation and phasing, and that means that getting your hands on the Necrons is not always straightforward. Simply beaming one straight into an assimilation chamber is not guaranteed to work. Being subject to a transporter beam could trip the Necrons' "teleport-home" feature.
However, the Borg will have an easy time getting face-to-face contact and attacking via assimilation tubules.
Step 2: Get nanoprobes inside the Necron.
If the Necron is able to be transported to an assimilation chamber, it can be held in place via force-fields/tractors and studied at leisure, so long as the Borg do not actually trip the "teleport-home" feature via gross physical damage. Transporters can actually be used to inject nanoprobes.
Otherwise, we have the question of whether or not assimilation tubules can penetrate the Necron living metal. This would seem very obvious. Necron living metal is "tough" only on the scale of plasteel and adamantium. We're talking about something that can get successfully attacked by weapons that have trouble with a similar or lesser thickness of adamantium - Space Marine armor - and chopped up by that kind of stuff.
Adamantium is five times as tough as steel. Tritanium runs somewhere around 10-100x as resistant as steel to various attacks. Given that Borg assimilation tubules (and nanites) don't have any trouble
at all with piercing Federation metals, we have that the Borg can get nanites inside a Necron.
Step 3: Compromising the integrity of the Necron.
First, what is the speed of Necron self-repair as opposed to Borg assimilation? Second, will Necron self-repair fix the Borg
alterations as if they are damage? Third, will the Necron "immune system" target and destroy Borg nanites, or will the Borg nanites target and compromise the Necron self-repair mechanisms?
I'm honestly not sure. I was just looking through a Necron Codex. Still not sure. There's something of a lack of concrete evidence on how the Necron maintain their Necron-ness, so there's some wiggle room here.
Still, we should have a concrete reason
why the Borg can't assimilate. They are generally able to assimilate 99+% of everything they encounter, whether it is more or less advanced than their own technology. Without a very specific reason why a basic Necron warrior has an artificial "immune system" on the level of Species 8472's, I don't see why they should be immune.
In general, Necrons lack immunity to special weapons in the game not just because of mechanical balance, but also, in-universe, it has proven relatively easy to adapt weapons to have an equivalent effect on Necrons:
Necron Codex, 3rd edition wrote:Necrons are largely mechanical creatures, and as such it might seem inappropriate that weapons such as sniper rifles, which generally use poison to achieve their effect, and agonisers, which work against an enemy's nervous system, should be effective against them. In practice, anyone using these weapons against Necrons would make adjustments to counter the Necrons' defences, for example, using acid rounds instead of poisoned rounds or altering the charge from an agoniser to affect the Necrons' power systems.
So it seems reasonable to conclude that the Borg can assimilate a Necron warrior. They have a strong self-repair factor, but that doesn't render them immune to the mechanical equivalent of a neurological retrovirus.
Step 4: Keeping your Necron drone.
Here's the problem. While it's true that WH40k fans on SBC are generally engaged in hyperbole instead of debate, we know quite well that drones can influence the Collective's behavior from within, and in some cases break free from the collective.
The Necrons are under a magical curse of sorts binding them to certain dark powers in the WH40kverse, namely, the C'tan. What prevents the C'tan from taking
back control of their newly Borgified Necron? What prevents them from taking over the Collective's group-mind, for that matter?