With the exception of Haven, Coryfishstick's temporary successes are only due to his victims being even more incompetent (Templars using red lyrium, mages siding with Tevinter, grey wardens siding with Tevinter). Even in those cases, he is thwarted at every turn, and in the end, he delivers himself to the Inquisitor's doorstep with almost no backup.
Grey wardens were not incompetent. Neither were templars and mages. Warden heard the calling and believed that they all had to start a long walk (one of their most important and sacred traditions) which meant the world would be defenceless when the next blight come. That made them open to cooperation with whoever was ready to offer help. Mages and templars fell victims to the fact that they spent all their lives in mutual distrust and fear. Corypheus used a deep understanding of how his enemies think to devise very smart plans to make them submit to his will. Compare it to Archdemon who only used darkspawns who had no other choice but to follow his orders and never made a single attempt to use any tactics other than "go forward, kill everyone you see".
Also, the Archdemon never showed up at Ostagar. Even if it lost that battle, it would have left the enemy forces devastated and demoralized, while it's own forces could, as far as we know, be easily replenished.
So he is competent because his forces are virtually infinite and he can waste them without any regard for efficiency or military strategy? What was the point of attacking Denerim, very defensible fortress before defeating separated enemy forces anyway?
Finally, you neglect the fact that Loghain and his supporters are also antagonists, and Loghain is a morally grey villain, while Coryfishstick is obviously evil.
He wanted power and he didn't care about anything else. How is he morally grey?