The jist of it is that in the original Necron Codex, the C'Tan ordered their Necron drones into hibernation to escape the threat posed by greater foes, wait for their enemies to die out and for life to return to the galaxy so they could rise again and resume harvesting.
In the NEW Codex, the Necrons rebelled against the C'tan. AND WON. WHAT?! How do you KILL STAR GODS?! And the new reason for going to sleep was because the Lords, which now have free will, and the abillity to speak english, ordered their drones into hibernation because the Eldar were too powerful to risk going to war against.
In essence, this completely Retcons the Necron Pariahs out of existance, which in turn null-and-void's an entire group of novels in the Ciaphas Cain series where Pariah characters are encountered and have backgrounds heavily grounded in the original codex.
But it gets worse. In the Blood Angels Codex, (ALSO written by Ward) the Necron's leader, the Spirit King, allies with the Blood Angels to fight off a Tyranid Swarm. And when the fight is over, they go their seperate ways like nothing happened. See, Tau Empire and Ultramarines allying to fight Necrons, THAT I can accept, but not this.
Ward is a blight on Canon. He just goes "Derp, I don't like this part, so I'll pretend it never happened and destroy an entire decade of established canon!"
I thought that might be a point of a contention, they're not as single-minded anymore. That Spirit King crap makes me facepalm, but whatever, at least in the novels it seems the authors ignore the terrible lore or re-purpose it so it's not as bad.
Yeah... Why does Matt Ward still have a job? He's like Nurgle Incarnate. Everything he touches, he poisons. I used to be a loyal Ultramarines player, loving their previous lore and fiction. Aftwe Ward defiled them with his fanboying (he is an Ultramarines player, and it SHOWS) making them practically second to the Emperor himself rather than ordinary space marines who do their duty and face the impossible, I had enough. I outright stopped playing Space Marines and turned to Chaos, taking up Red Corsairs rather than endure such humiliation and mocking from fellow players.