Yes, Draigo is a Grey Knight, and an accomplished one at that. That does not change the odds against which he would face against such powerful entities as Mortarion, or the daemonic entities within the realms of the Gods, or surviving the Warp in the first place where other mortals simply cannot survive. What's more, nothing has been presented that could alter the odds of survival; many have provided explanations, but Matt Ward embraces none of them.
It took 109 Grey Knights to tackle Angron and his Bloodthirster bodyguard in the First War of Armageddon, and only 13 survived. As for Draigo vs. Mortarion?
Originally Posted by The Battle of Kornovin, 901.M41
Alone and unaided, Draigo smashes his way through Mortarion's bodyguard, strikes the Primarch to the ground and carves Geronitan's name on the Daemon's vile heart. Though Mortarion ultimately escapes, it is many long years before he can enter the mortal realm once more.
There are things we don't know, of course. We don't know about any prior fighting beforehand. We don't know how powerful Mortarion's bodyguard was. We don't know Mortarion's prior condition. Can we expect [satisfactory] answers from Matt Ward himself to these questions? Consider this, found on Draigo's page on GW's website:
Originally Posted by Matt Ward
Lord Kaldor Draigo is a combat monster - there's no other way to describe him. He's lethal against non-daemonic foes, with plenty of Strength 5 force weapon attacks to lay a beat down. When faced with hated Daemons, his Titansword becomes Strength 10, ensuring a pretty one-sided fight in his favour. Even if his enemy survives, Draigo's storm shield is sure to keep him fighting. And on top of all of this, Draigo is a Grand Master, able to bestow extra abilities on his allies. Want your Dreadknight to capture objectives? Draigo can make that happen. Want a Scouting screen of Dreadnoughts? Draigo can make it happen. He's the best possible way to keep your opponent on his toes.
Matt Ward has a history of shilling whatever faction it is he's working on; his comments on the Ultramarines in a White Dwarf article (can't remember which) are well known by now and do not bear repeating. This has given him a reputation in the eyes of many readers as a tool and a hack. Are we to accept at face value that Draigo defeated Mortarion effortlessly while the latter was at the pinnacle of his power?
Of course not. You can ask readers to accept the impossible, but not the improbable. 40k is full of impossibilities, but the setting must stay consistent with itself. That is why readers such as myself take issue with Draigo's battle against Mortarion and his thriving in the Warp - because both are improbable, and nothing has been done by Ward to even the odds. Couple this with Ward's percieved attitude and readers like myself get the impression that Draigo's achievements have been tacked on to increase the size of his proverbial dick, to make him look awesome.
Yes, Draigo's story is meant to be tragic - he can achieve no lasting victories against his daemonic foes, and he is trapped in the Warp for ten millennia. That point, however, could have been reached without such suspension-of-disbelief-shattering clutter. It's like squeezing a lemon with a Rube Goldberg device; sure, you have your lemon juice, but couldn't you have just as easily squeezed the lemon with your own hands?
To sum up, some readers don't like Draigo's fluff because they don't believe it