Marshall was way too sanguine about Allied capabilities, and didn't understand the difficulties of amphibious operations. The Sledgehammer plans were totally unrealistic, calling for a landing on the north side of the Brittany Peninsula, followed by a quick thrust to capture Brest.
There were numerous flaws in this concept. First, the north coast of Brittany was not conducive to amphibious landings.
Second, as the Morocco landings proved, the US Army was not capable of carrying off such an operation.
Third, Sledgehammer would not have been a simple overnight shore-to-shore operation like Overlord, but would have required at least two days at sea in U-boat infested waters (remember, Lorient was just around the end of the peninsula).
Fourth, the Allies wouldn't have been operating with anything approaching aerial superiority.
Fifth, the post-landing logistics would have been a nightmare. The Germans were quite capable of rendering Brest harbor unusable, and the Mulberries were not even in the planning stages.