In both WW I and II, US internal security benefited from a wave of patriotic sentiment that bolstered the loyalties of German Americans. In addition, before WW II, the Roosevelt Administration had cracked down hard on the Bund and shut down various German spy rings before the war.
Even at that, the Germans had some notable early espionage successes in stealing plans for the Norden bomb sight and for the main US submarine torpedo. Like the British, the Germans were unimpressed by the Norden, and they detected and corrected the defects in the US torpedo firing mechanism that bedeviled US submariners during the first half of the war in the Pacific.
I couldn’t imagine the security issues involved as we went to war with countries that had fed our “melting pot”; of course, it helped that back then there was no political correctness. If somebody’s last name indicated they MIGHT be a security threat, they were simply kept away from sensitive material (without fear of a lawsuit).