Because, yes, there was a problem with that they were supposed to rival other Gods, however on the table were pretty shitty. My problem lies that in the past they were supposedly godly rape machines, but with them being turned into shards and being enslaved by Necrons, they got turned into glorified Pokemon.
When they came out they were the heaviest hitting things that weren't apocalypse, but power creep and all they didn't get buffed into an apocalypse equivalent.
C'Tan sharding really fucked up the lore; before the C'Tan were the realspace equivalent of the Chaos Gods, where the remnants of the War in Heaven were threatening to alter the very nature of the entire galaxy. The Chaos Gods wanting the immaterium to usurp the material, the C'Tan wanting the material to usurp the immaterium.
They were hyped as these world-ending entities that were amassing their forces for instant plot-breaking apocalypse, things like the assault on Mars or the World Engine being only the beginning. The theme that most defined the necrons was their implacability, their awakening and their mastery. Implacable, they continually advanced (in lore and on the table). Mastery, as they had mastered the material universe, capable of FTL non-warp travel, their basic voidships were more powerful than some capital ships (and their own capital ships like the World Engine or the Nightbringer's capital ship are god-killing). Their rules reflected this with things like the basic necron gun being capable of taking down a land raider. The last was awakening. The typical necron story was some faction begins stumbling amongst some ruins and awakens the slumbering evil, that doesn't sleep, doesn't eat, can't be bargained with or reasoned with - think that speech in terminator voiced over in sick perturbator beats. The machine that had a soul, awakened, coming for you. The worst part being that these soulless automatons were only the vanguard, that the true awakening was to be their Gods, the final masters of the material.
The Deceiver was awakened and sowing the seeds of discord within the Imperium.
The Ultramarines awakened the nightbringer and narrowly stopped the nightbringer from ending the Imperium, but could not stop it from escaping.
The Outsider was never developed on but was potentially the Tyranid swarm.
The Void Dragon by far was the absolute worst one, it would have actually brought the plot into the next milennium if it awoke.
All this awakening, all these world-ending threats, and the one hope was that they remained slumbering. The horrifying thing was that... Well, you know those Lovecraftian stories where the good ending is that no one notices you and lets you kill yourself in peace? This is not one of those stories with a happy ending. Not only have the C'Tan noticed humanity, but they are one of the four entities that have
decided the course of human evolution. Imagine finding out intelligent design was real, and pantheon of the Gods who designed your predecessors are dead, the God you believe watches over you is dying, leaving only two groups fighting to claw over your species to ensure their total supremacy. If humanity cannot fulfill the Emperor's final wish to evolve into a race of enlightened psykers capable of avoiding the Fall of the Eldar, they will surely become the playthings of these Gods.
Would you choose to become the material serfs of the C'Tan, or the gibbering torment of Chaos?
That was the best thing about the Cron lore really, how it put things into scale. The War in Heaven fluff is easily some of the best lore 40k has to offer, and it gave the Eldar some much needed time in the sun showing just how high they were and how far they fell. Alfabusa's shorts really capture it well, that the 10,000 conflict between the Imperium and Chaos seems to pale in comparison to the wars between the C'Tan & the Old One's successor deities over
reality itself.
3rd edition onwards lore was about discovering the truth, and attempting to cover it up - genuinely for the good of everyone. Take for example the Mechanicus or Eldar, who are very guilty of hiding their knowledge because it's valuable and conserving that knowledge from everyone else maintains their power. This is one case where you get Mechanicus agents ripping out all of their mechanical implants (when you consider how they become more machine than man, that is saying something) or Eldar slaughtering entire Imperial planets just to try and keep the awakening from happening, because no one has the strength to combat them directly anymore... Not without turning to Chaos for help.
Cadia is a great example of all this happening, how this lore added to everything else without obscuring it or detracting from it. You had the Imperium and Abaddon playing with Blackstone Fortresses and Cadian pylons completely ignorant of how these weapons were meant to end Chaos gods or C'Tan
permanently, with Eldar and Necrons showing up to try and manipulate either faction into blunting one another. Really helped the Eldar not seem petty too, the lesser races really were fucking things up big time and they didn't, couldn't and wouldn't understand, because to confront and realize the full situation they would have to admit to the foulest heresies against the Imperial and Mechanicus creeds.
Thus, the appearance of the Crons in lore was the theme of awakening. The Eldar were ancient foes of the Necrons, desperately trying to stop the Monkeigh from awakening the slumbering immortals. Chaos were ancient foes of the Necrons, manipulating their minions into thwarting the plans of the C'Tan to gain total supremacy over the galaxy's souls. Orks felt something
ancient stir in their very DNA at the sight of the Necrons, krumping was soon to occur on a galactic scale. The Inquisition found itself having to suppress a new heresy, a new threat, even new dimensions of reality to deal with. The Tau attempts at making friends with the Necrons ended in harvest. The Dark Eldar recoiled in horror as they realized their old foe, one capable of destroying even their smallest cell - could return, causing them to be harvested by Slaanesh or C'Tan. The Nids gave a wide berth between Tomb worlds and their fleets, and the Imperium noticed.
Given how all the lore was building up to awakening, you'd think the next update to the Necron codex would be that: The awakening has begun. The plot moves forward, everyone has met the Crons, now it's time to wake up and emerge.
Instead the C'Tan were broken up into shards, the Necron threat was broken up into sub-Empires and control handed over to Tomb King pharoahs in space.
Don't get me wrong I like the Pharoahs it just seems like so much of plot was undone for no reason, or worse, done because it made the Ultramarines look bad.
This is made worse by how Matt Ward doesn't get epic, nor horror, nor how cosmic scale and horror align.
The wraiths, the ethereal surgeons who phased through walls to ambush your units and flense them apart, before disappearing through solid walls again? Gone.
The pariahs? Need I say more?
And the C'Tan. Now they're pokemon.
Crunchwise the 3rd edition rules captured their feel well. Their units were OP on purpose, most of their units were slow and they played like a rolling train of Napoleonic redcoats, steadily advancing forwards with an emphasis on mid-range heavy firepower, augmented by easily the best unit in all of 40k: The humble scarab. Their one weakness is that there wasn't really much in the way of ways you play crons differently, once you and your opponents realized the best tactic was just SPAM WARRIORS AND ADVANCE PHALANXES there wasn't much more you could do with them (besides cloak of shadow Necron Lord + deep necron warriors deep striking all over the board for shits and giggles). Actually, deep striking was one of the best things Necrons could do, it was always fucking hilarious deep striking monoliths into the heart of enemy formations. The Necrons were kinda crippled by their rules, since they had this phase out rule where if only 25% of Necron units were still alive, they phased out and the enemy won no matter what the victory goals were. This was supposed to balance out the Necron's strengths by making it so opponents only had to kill 3/4 of them, but all it meant was that Necron players had to max out their necron warrior model count otherwise they could be tabled by players maxing pie templates or CQC models with power weapons.
5th edition Necrons played like Monoliths: The faction. Basically you put Necron warriors behind Monoliths and just put Monoliths everywhere, Monoliths everywhere!!! It was kinda a hassle actually, as having so many Monoliths was a trouble to transport anywhere. Monoliths were larger than Apocalypse tanks and thus fucking impossible to transport anywhere. Needless to say indestructible giant bricks deep striking everywhere, while hilarious, was probably not in line with cron fluff - indestructible giant bricks deep striking everywhere is part of the fluff, but never are deep striking giant bricks the entirety of the cron invasion. Scarab swarms as usual the best unit ever, a swarm of skimmers that move 12" and are relatively useless especially if they eat pie templates. However if they make it to the enemy tank line, they begin eating the tanks up for breakfast.
6th edition Necrons have many, many more options, but their cheese is broken as hell and monoliths were nerfed so that people would have to buy new models. Crons made much more shooty, scarabs nerfed, necron resurrection abilities nerfed but phase out removed to compensate. Focus now not on spamming baseline infantry or superheavy tanks, but on surgically destroying enemy heavy weapons, fliers and HQ systematically. Plays a lot like if you crossed dark eldar with spehss mahrin now, with baseline infantry being heavy infantry with the fast-moving skimmer transport options typical of DE. Overall, the Crons got more powerful and options available in 6th ed than any previous ed
Definite emphasis on remaking them from the automaton harvesters of lesser organisms and more tragic emos. They're not a plot-advancing galactic threat, they make alliances with blood angels and shit, turns out you can negotiate with the things you can't negotiate with. Their gods aren't even gods anymore, they're about as threatening as avatars of Khaine.
Makes all that fluff buildup seem pretty worthless now :\