Well, if the Germans don't occupy it on the last round, the Americans might just walk in, esp. should they have the critical objective card, which gives them extra movement. So a savvy German player will do that anyway, I feel.Remember, as you asked, the US win immediately the moment they occupy the objective hex, so no chance to counterattack.
Funny you should mention the extra round. While designing the scenario I was indeed doubting between 4 and 5 rounds all the time. I wanted to keep the scenario as fast and furious as possibile and therefore went with 4 rounds in the end, but I might as well have gone with 5. I think that will often give the US just that extra bit of time they may sometimes need to pull off a win, esp. when they DON'T have the critical objective card in hand.
Thanks for playing!