I never thought of Nightfall as flawless. Venka, yes, but not Nightfall.
I guess it's because the first time we see Nightfall, she's confused, unsure-- she asks Cutter why the humans hate them so. Then we see her in the desert with dying Redlance-- very vulnerable, very shaken.
Vulnerability. Yes, that's the thing Nightfall shows much more readily than Venka-- though I suspect both characters have it. But where Venka seems completely self-possessed and in command of herself, Nightfall occasionally comes across as frightened or unsure.
I also suspect a tendency to a warrior-like brashness which Redlance helps her balance.
Nightfall seems to need Redlance. Venka, by contrast, doesn't seem to need anyone.
Don't get me wrong-- I don't dislike Venka. I'm just using her to contrast with Nightfall in this area of alleged perfection.
I see Nightfall as warm, generous, brave, accepting and wise-- she'd make a good best friend. But not perfect. :)