The thing is though, Draigo accomplished precisely nothing in doing so.
None of his actions have a lasting impact because he's in the Warp. So sure, he's going around roughhousing daemons and knocking down their buildings...but then they're just like "that ******* again? Damn kids..." and things go back to normal.
So I really don't get why people are that upset over it...
The only part of it that doesn't sit well with me, personally, is that Draigo tagged Mortarion's heart. Burning Nurgles jungle, though? Or knocking down some buildings in the Impossible City? Sure, why not? Everything just resets itself after he's left, and the daemons go back about their business.
I wouldn't call it bad writing. He's done nothing to diminish the threat of Chaos. Absolutely nothing. The only time he ever does any legitimate good for the Imperium is when he slips back out into the material realm again...and then he's just another Space Marine hero. I think the only reason he can do any of those things in the Warp in the first place is because he's an insanely powerful psyker and he's in a realm of psychic energy and willpower to begin with. Add to his prodigious psychic ability the fact he's encased in the best armour and wards the Imperium has for its psykers, and it's easy to see how he remains free of the Warp's malign influence. Terminator armour enables Marines to teleport through the Warp in safety, remember.
In the material world, though? Powerful psyker, sure, but Draigo's not toppling buildings and torching jungles there.
Just doesn't seem like a big deal to me, or that out of place, really. Maybe I'm just more tolerant of changes to the fluff than most people.