It's not as bad as when they published the Daemons codex, where the Chaos Gods were essentially reduced to Greek god knock-offs with very little mystery around them (it's especially odd hearing the Chaos Gods referred to as though they actually had physical bodies and were intelligences similar to a human mind and see it presented as some kind of objective truth.
The Black Crusade books are looking to be heading in that direction as well I'm afraid. We get glowing descriptions of where Tzeentch supposedly lives in the Warp and details of what happens to people who try and visit It. I'm with you where I prefer the Chaos gods to be very nebulous and not some guy you might run into down at the pub.
The current generation Daemons of Chaos codex (though outdated a bit) and the Grey Knight codex both imply that they nature of Chaos is still very much the spanning nebulous entities, and that the images and temperments associated with them are less their true nature, and more the human minds vain attempt to rationalise them. We SEE Nurgle as a fat old man sitting over a giant cauldron because we cannot cognitivly understand what he (or more appropriately, IT) really is. I haven't read the
Black Crusade books however, so i can't say how much of that explination continues to hold up.
If they are going more towards the literal, rather than interpretive perspective on Chaos, then i can't say it's a particularly attractive approach. With luck, they'll have a moment of clairity, like Bethesda did after Oblivion, and being the metaphysical shenanigans back.
But i digress... Regardless of the approach, i think the move away from the whole 'Chaos Undivided' approach is a good one. 'Undivided' represents the 4 powers working in concert, but because of their individual natures, and the natures of their champions, them all blessing individuals with their power has always been unlikely. Instead, i think it more appropriate that Chaos Undivide be represented by forces of each God, working together but remaining evidently seperate. The obvious exception, of course, being imensely powerful champions of Chaos, like Abbadon, but their in another weight class altogether.
The idea of Chaos functioning as a working pantheon which can be worshiped in its entirety has also never, in my mind, really meshed with the nature of how one falls to Chaos. Worshiping 'Chaos Undivided' would have to be a concious decision, but people fall to chaos before even realising it. By the time they are aware of the actual circumstances of their damnation, their individual temperments have already brought them in line with a particular God. This is how individuals like Angron, Magnus and Fulgrum fell to their respective gods without even knowing it.
Worshiping Chaos is less of a choice and more a reflection of an individuals own nature, and that nature enevitably pushes one towards a particular god before any willful decision is made, rendering the notion of Chaos Undivide inherently moot.
Of course, that doesn't change the fact that there are still other types of warp entities not nessessarily born of the 4 powers. And we know certian types of Daemon are independant and willful enough to change alegiences over time. Still, thats all an issue more for the nature of the Warp and less about the religious implications of Chaos.