Yup. I kinda agree. For me, it was the bloodletters that were convincing me that it was fake, but good point with the dance of death power. That would basically be auto-win against many Necron and Tyranid players. (A 'nid player could probably survive losing control of a carnifex or tervigon, but wouldn't be happy. But a wraith unit going rouge in their back lines? Yeah, that could wipe him out, quickly)
I don't have a problem with the Nugle Demons stats. 6 Wounds is pretty common now, and Iron arm already makes it wasy to get it to T8. It's slow though, not winged, so you have plenty of time to bring it down. I don't see a problem.
That blood thirster though...basically no way to kill it in CC. Which is OK, good even, it's fine for the swarmlord. But bringing down a FMC before it gets to you is very luck dependent. You'll either get lucky and bring it down and he'll have wasted 280 pts, or it'll kill your whole army. I don't see too much of that as skill dependent.