As a Scholar, the only thing I hate about DRK is dealing with Living Dead because it'll probably eat up at least two of my Lustrates and Emergency Tactics. Other than that, there is nothing DRK can't do that the other two tanks can't, specially in trash pulls. As someone else said earlier, if you're using Dark Arts and Abyssal Drain when Blood Price is up you shouldn't have much trouble keeping yourself up.
If your point of comparison is Main Scenario roulette... I'd put the blame on healers who get overconfident and don't pay enough attention cause they're used to just steamroll through that particular dungeon. If your gear is not capped in stats they might not see a lot of incoming damage coming, but you also said someone else told you you're the tankiest DRK they've met, so...
I think you've just had bad luck with Duty Finder. My suggestion as a fellow tank player is that you use the first one or two pulls in a dungeon to gauge the healer you get, and then adjust your playstyle accordingly!
