I might agree that for YYZ-CPH, you're better off with YYZ-CPH on AC J than YYZ-ORD-CPH (I don't know if that exists, but you get the idea) on UA F.

But in DrunkCargo's case, he's ticketed in F, he isn't exempt from AC YQ, and there's a non-stop IAD-FRA UA flight.

I found UA F service to be similar to AC J. The product, however, was better. So if I'm flying TATL on an F ticket and there's no LH available, UA F is my preferred fallback.