There's one thing that people need to realize when it comes to demon primarchs..... Yes they are insanely powerful, yes they are even stronger then greater demons ( in most cases ) but the problem is in the fact that they are demons.
Primarchs in my opinion actually got weaker when they turned to demonhood. Everything in Grey knight's arsenal works against them, especially when we are talking about a Grandmaster.
Now, if this was regular great crusade era Angron, or any Primarch, then any of them would have ripped apart Draigo like he were a guardsman.
Besides, Draigo isn't a thing now anymore. Too bad Epimetheus is now trapped by Abadon because I would love to know just how powerful this guy is. He's one of the founding members afterall. Draigo is a brat for him.