I found Illyria to be chilling as well, but mostly because of what her existence meant in terms of the transformation of Fred (the episode in which it happens was nothing sort of gut-wrenching - I'm not normally easily emotionally disturbed, but memories of that episode are very actively unpleasant to this day). I don't think Acker acted her all that well - I just don't buy cold and evil from her.
Well, your mileage may vary, but I literally don't know how you could say that. Chalk it up to different aesthetic tastes.
As for Strahovsky, I actually only have her voice to go by, to be honest. I have only vague memories of the Chuck pilot at this point and never watched more. But she voices (and lends her physical likeness to) a key character in Mass Effect 2 who is trained paramilitary, and even then doesn't seem particularly hardassed.
I can't speak to anything related to Mass Effect