I'm not sure about the "as practiced in aikido" part.. i seem to feel differently about my practice.
Koshinage works fine for me if I can get low enough, mantain the correct posture and am close enough so that it fits the dynamics of the movement.
but alas, no move is appropriate all the time, so sure enough its very possible to have it countered if the opening isn't there and you try it anyway.
I like koshinage because its a challenging technique to learn, as nage and uke. I think the fear of ukemi from the possible variations of this technique is a probably a reason its not seen so oftern in class.
But if you're in close, and feel the momentum, it works all right for me.