Non-darkspawn have never managed to win any meaningful military victory over an entire horde.
I think that they have. For one thing, Drakon's Orlesian armies scored several notable victories over the darkspawn in the Second Blight. These victories helped him cement his power and authority in southern Thedas, even though the Blight itself was not ended: indicative, I think, that the darkspawn menace was not necessarily an existential threat at that time. Earlier, in the First Blight, Tevinter armies began turning back the darkspawn long before Dumat was killed the
first time, and after Dumat was killed the
last time, the horde remained in being, albeit badly disorganized, and had to be forced underground in a series of hard-fought campaigns.
But on a more general level, if the darkspawn won chains of endless victories and nothing but until the Archdemon was slain, it's hard to imagine the civilizations of Thedas
surviving at all. Defensive wins, like the Tevinter victory in the siege of Minrathous during the Second Blight, are plenty "meaningful" on their own.
Ultimately, though, battle is a lottery, and darkspawn armies can lose that particular game of chance just as much as any other armies.
Where I agree with you, though, is that intelligent darkspawn improve the odds of any horde's success to the degree that it's very easy to argue that they should not be allowed to exist - if one wanted to argue that. And since darkspawn are literally impossible to coexist with, due to the corrupting nature of the taint, it's very defensible to suggest that it's not safe enough to simply trust in their intelligent leaders to 1) not fight and 2) keep away from everybody else.