I tend to play my Black Legion armies with a focus on black legion marines, built around mechanised formations. I use some armour (chaos preds and land raiders in seperate formations usually). I have tried a range of army styles though. The playtest changes havent significantly altered my style of play (although they have made it easier to leave out terminators/foot retinues as there is flak elsewhere)
Defiler - Not used. I would consider using it now though, if I had some figures!
Desecrator - see above
Hellblades- these are my AA of choice. I've found them good when doing AA but of limited use in ground attacks unless they find some juicy infantry targets. Underpriced? Possibly, but I don't think my games give a good reflection as they are usually aircraft light.
Obliterators - I've still taken 1-2 obliterators in terminator formations to try them out. They have certainly given terminators the chance to put out a bit of ranged fire after the first assault, when everything near them is dead

Chaos terminators struggle to do this due to their low AT value otherwise. Ive had obliterators take out several manticore formations this way. In my opinion this is what you pay for, although I would say they do seem a bit overpriced now. (by how much, I couldnt say)
Ferals - MW+IC Ferals were overpowered IMO. I used 3 once, I think, as I almost felt like it was cheating to use so many - IG mechanised infantry companies disappeared like chaff, and there was little that could be done to stop them with clever use of terrain. I'm not sure, however, if without MW they are actually worth 300 rather than 275. Yes the death storm is better than the VMB, but twinning that with the Hellmouth gives substantially less AT firepower overall in real terms. Firefight is not in my experience an option due to low effective formation size - anything a feral can firefight, it can shoot better.
That said I can't say they have underperformed at 300
Vindicators - Not used
Greater daemon - Not used in Black legion
Ravager - Ive actually had good use out of the ravager in history, but mainly as a FF platform. The TK gun is rubbish and needs a boost IMO, as the titan does feel undergunned.
Raptors - I use them at 35, I use them at 40. I feel like I am getting less of a bargain at 40, I'm not sure they are underpriced. The 4+ firefight is very nice, but they are easily easily broken in my experience due to smallish formation sizes and lack of tanks to hide behind.
Factions - Works fine
Stalker - Used this quite a lot, it seems fine. If anything a bit overpriced - as I mentioned earlier our games are light on aircraft so that may be just a personal perspective.