I'm hearing a lot of possible explanations for why the Wardens fared so poorly and the Inquisition did so well, and while I can kinda understand the losses due to the sacrifices/infighting in the ranks, the notion that the Wardens are incapable of open warfare doesn't make any sense, given what we've seen previously in the franchise.
And that's the major problem right there. The Wardens have been established as wholly capable from day one. From the intro cinematic of Origins and beyond, the Wardens have always been portrayed as excellent fighters and a respected military order for a reason. That doesn't mean they were invincible, or that they couldn't have moral ambiguities, or even that they always won, but at the end of the day, we were given a clear picture as to what exactly they were capable of, and how they could stand against enemies as monstrous as the darkspawn.
Conversely, the Inquisition's military forces are never effectively established as a genuine powerhouse, let alone something that is superior to both the Grey Wardens and the militaries of both Ferelden and Orlais. We're shown that the Wardens are badasses. With the Inquisitorial troops, we're merely told.
(As an aside, I think that's the general problem with the Adamant arc as a whole, nothing is effectively established; not the superiority of the Inquisition troops over the Wardens, or the danger posed by Nightmare, or the need to sacrifice someone to delay it. Everything happens because the plot says so, not because it makes any real sense).