Duncan helped only the people whom he wanted to recruit or use and while i may argue that Duncan's desire is only the best interest of the world and that he is doing this only for the world i dislike this modus operandi as well which is not different than the one adopted by Flemeth.
The "i help you only because i need you" sorry i'm not an object and i do not desire your help if it is only meant to benefit from me much less if it is done with dishonesty both Duncan and Flemeth.
I don't see why Duncan is being dishonest. He one of the few characters that is really honest in the game. He outright tells that he recruited you because he though your abilities are an asset to the Grey Wardens, regardless of whatever you are or think about him. "That conscripting you saved your life is only circumstance", it's something he can say to every potential Warden at the end of their Origin stories. He is no convincing you with lies or that honor stuff Alistair and Fakewall prattle about, and can even say you that, is you're are a thief it doesn't matter as you don't get caught. And can even admit that the Joining is potentially lethal if you ask.
You can disagree with him and his methods, but Duncan is one of the few honest people in DA:O.
I ask him are their any other surprises? No, just dying young and defeat the Blight alone. But there are surprises, ugly ones. For males it is simply ghouldom. For females it is the potential to be a broodmother, and this after he attempts to seduce you.
There is the possibility that he didn't knew about that. He was, after all, only a junior member of the Orden during the Fifth Blight; and as Stroud/Loghain/Alistair say in DA:I, there is stuff among the Wardens that only is revealed to a very few senior Wardens (like the existence of Corypheus, the truth behind the Calling, etc.). It would be obvious that something so cruel like the existence of Broodmothers, is keep a secret to the junior Wardens.