I honestly seem to be alone in not really caring for the Crimson Slaughter Possessed--if you could pick and choose which bonus you get per turn (say, like Oblits and Mutilators), the ability to a) speed up (if not in a vehicle), b) get cover (even if in a vehicle), and boost combat abilities might be nice--but as it stands, you have no control over when you are in combat gaining Shrouded or turning into Beasts, and that's 2/3 of the time no combat buffs.
The demerits for standard Possessed are that they are: over-priced, fragile, lacking combat grenades; and lacking mobility. Randomly helping out the fragile and lacking mobility, which are situationally needed, rather than reliably getting some sort of close combat buff... strikes me as inefficient. I can build around their slowness and fragility, not still have to build around it but hope for whichever 1/3 chance I need that turn. If I get them a Fire Raptor as a transport who cares if they turn into Beasts that turn? If they're stuck in combat who cares if they get Shrouded?
I want reliability, not flukey partial compensation for their flaws.
Anyways, Possessed are... not great, on paper. The best use I've found for them is as the second punch in a one-two wave in my Khorne Daemonkin army, since they're a necessary part of a Slaughter Cult formation. When I have 8-odd squads of minimum-strength Bikers and Flesh Hounds, a full Goretide, zipping forward, most of the time my opponents don't really have the shooting to also focus on my 5 Possessed (with naked lord with Bloodthirster-summonging axe in) and the 2 squads of 10 CSM who disembark from their Rhinos after running forward to hit in a second wave behind all the Maulerfiends, Bikers, Flesh Hounds, etc. When your opponent is juggling too many other targets to shoot at, and the Possessed have both Rage and Furious Charge, they pack a surprising wallop against anything that isn't in 2+ armor.
*is not going to rise to the flamebait about AD-B, who is certainly in the upper tier of BL's authors. But then, also my favorite BL author is Matt Farrer, so clearly take my opinions with a grain of salt*