I have found a partial answer in the archives.

In 2002, in <[www.uboat.net];, Kurt described that the US boats used a Diesel-Generator, and Motor-Prop lineup instead of the direct drive on 2-engine U-Boats.

The US scheme required an additional electric machine but avoided many vibration issues, especially when the sub was four-engine, not two. This confirms my speculation re: shaft alignment being an ongoing issue. Plus the Diesels do not need reverse. [I also see reverse was removed from some U-Boats Diesels when they got a Schnorchel added.]

That seconds machine did need space and copper. The Allies were short on copper [but not as short as Germany...] so in one amazing case, the Manhattan Project, they borrowed tens of tons of silver from the US Mint.