I have always liked that about the Chaos Marines; that many of them are 10,000 year old veterans who fought in the original Horus Heresy. The bitterness and spiteful infighting in the aftermath of that is what has kept them down for so long. Abaddon can call all the Black Crusades he wants, and wear Horus' lightning claw, but he isn't Horus, and has never united the Traitor Legions like the Warmaster did. So they putter around, raid the Imperium, and fight amongst themselves. As destructive as their raids, wars and Black Crusades are, they have never really come any where near destroying the Imperium. They are written to be exactly like the fallen angels in Paradise Lost; they lost, and now all they can think of to do is to tear down the Imperium that defeated them, forever.
The fact that they have to hide in the Eye, where the Warp bleeds into realspace, has allowed them to survive and continue this war. Time doesn't pass in the warp like it does in the material world, so like ships traveling through the warp, the Chaos Marines timeline is vastly different from the Imperium's. In the Night Lords novel Soul Hunter, for at least that ship, they're only a century from the Heresy. The main character is an Apothecary, and he talks about the need for recruitment and the difficulties inherent in it. Geneseed is corrupted by demonic taint and mutation, which has become widespread through the traitor marines. He also mentions how few have the knowledge and expertise to create new marines.
I happen to love the concept of the Night Lords, as well as the novels starring them. Traitor Marines who mostly scorn Chaos, and view becoming enslaved to the Dark Gods and daemons to be a failing. Their principles have also weakened them, compared to the other Chaos Marines. They can't make up for losses and deficencies in their forces and supply lines with daemons and other profane instruments, so they're stuck scavenging the dead for power armor parts and tending to ever diminishing numbers of vehicles and manpower. Their viewpoint, as being the last righteous rebels, is a maudlin and interesting one. While still fearsome terror troopers, they still try to maintain their pride in their, now effectively defunct, legion. They haven't broken up as much as the other Legions, but they know they don't really exist as a unified force anymore. And what kills them is that's all they have left. They haven't fallen to Chaos, and exchanged one slavemaster, would-be god for another, but it hasn't kept them together anymore than the other legions.
They show how dangerous Space Marines really are though. Even bitter, broken and divided, they fight on and inflict terrible damage. 10,000 years of exile in hell, and they return again and again for another war. They just keep coming. Cut off from the supply lines that loyalist Chapters have, they steal, scavenge and bargain with hereteks and the Dark Mechanicus for wargear and technical assistance. Even then, the Imperium's technology has increased with time as more STC's were discovered and deciphered, while the Traitor's technology, with the exception of magitech devices, has stagnated. So they make do, and improvise daemon weapons like the Defiler to fill in their gaps. Their allies' have become unreliable at best, and zealous psychotics at worst. Actually, becoming possessed and entralled by daemons at worst. But they soldier on, and continue to attack, like Space Marines should! Because, as Konrad Curze said, death is nothing next to vindication.
Besides just Night Lords, the other intriguing thing I've seen about the Chaos Marines in Black Crusade is the Renegade archetype. What does a Space Marine do, when his Legion is dead or broken, and he has no more desire to stay with them? Marines are defined by their Chapter and Legion. It is community, vocation and religion to them. When one walks away from that, into the wastes, with only the wind and whispers of daemons for company, what motivates you?