It's one of my all-time favorites as well.
About the only thing I can say against it is the idea of getting in and out of Nancy in the summer of 44 is highly improbable. It makes the movie all that more exciting from that angle. I had a best friend that was there. He went in on D-Day +14 and spent most of his time fighting for his life until the Fall.
I also had a cousin that got sent in as a G2 operative to ferret out looting. He would be assigned as a general's driver and stuff like that. He ferreted out gold and art heists during and after the war, before moving over to CIA. They were actively looking for stuff like this, and even if these guys made it back to the lines, there would have had to be hell to pay to get the gold out. This heist left to many people in the know, and too much evidence pointing to the perps. However, the perps (Kelly, et al) would not have known any of this. How much of it was going on? A lot, but most of it was Colonels up to Brigadiers trying to ship stuff back to the states. My dad had a C/O, a colonel, that got caught shipping jeeps back to himself in the states. All those perps went to Levenworth.
Could it have been successful? Allies were on to this sort of thing from before the landings. The German situation is more chaotic. There was quite a bit of gold that disappeared that was never found. Most of the successful heists were Reich sponsored and the goal was to smuggle and hide stuff for the purpose of reconstituting the Reich later.
Could a German/American coalition have done it? Highly improbable, but if you leave out hindsight, the right people in the right places at the right time, it might have been attempted.
