>>718539525
3rd Edition Necrons were essentially Terminators in space. They awake from underground tombs, march out as a silent, soulless, nearly unkillable horde, kill everything down to bacteria.
5th Edition Necrons were given special characters, lots of new units and actual stories and lore beyond "wake up, kill everything". The already-present pseudo-Egyptian aesthetic got upgraded too. Flayed Ones got an actual background instead of "robots draping themselves in skin to look scarier". Lots of good novels featuring NuCron characters, too.
To me, it all links perfectly together chronologically. 3rd Edition Necrons were the first to awake, the worst functioning ones, with their memories almost completely erased. Unwashed, rusted horde of animated metal that only remembered how to kill. No one who encountered them knew who the fuck they were or why they did what they did. Then, actual rulers and the nobility awoke, with their memories nearly intact, along with old grudges and butthurt toward each other, and decided to take out their shiniest toys to play.
WH40k really didn't need another faction of "mindlessly kill everything", it already has Tyranids as a perfect galactic boogeyman. Even Orks and Dark Eldar are able to engage in diplomacy from time to time.
Secondaries screeching about muh nucrons can go eat a dick.