>>96267678
He's not only one of the most powerful psykers in the galaxy, he's specifically an anti-daemon psyker. That means it's perfectly fine for him to exert the power he does over the Warp, a dimension entirely based on being influenced and exploitable by psykers. A psyker like Draigo, with GK training and unbreakable belief in the Emperor and himself, would 100% bend the Warp around him to his will, it just goes back to the way things were when he moves on. Absolutely nothing about this is out of line or deviates from canon.
>but he beat
Yes, he defeats powerful daemons, consistently. Almost like he's the chapter master of the anti-daemon chapter and a post-human super soldier in addition to constantly radiating daemon kryptonite at all times.
>but Mortarion
A daemon. Which makes him vulnerable to Draigo, far more so that an equivalent enemy of equal potency, Primarch or not.
>but Nurgle plagues
A virus is a living organism. That makes the plagues Nurgle makes, in the Warp keep in mind, teeny tiny daemons. Meaning they're more likely than not to burn to nothing just getting close to someone like a powerful Grey Knight. As for more mundane plagues, he's still, you know, a Space Marine.
>but the last chapter master died from
He wasn't as powerful a psyker, was he? Not all chapter masters are created equal. In fact to the best of my recollection no chapter masters but Draigo are psykers at all. So, he's the most powerful Space Marine in the galaxy. And again, he's especially powerful against daemons. So his victories against them are perfectly fine. Now, if he was doing this shit against C'tan, Szarekh or someone similar, then yes, you would actually have a reason to bitch. But as things are, you don't.