Here’s a weapon which could more properly be described as “low-profile” rather than underrated: the proximity-fused anti-aircraft artillery shell. Proximity fuses were essentially miniature radar sets, built small enough to fit in a shell and tough enough to withstand the tremendous kick of being fired from a gun. They were quite a technical achievement and they increased the chance that a shell would bring down an airplane even if it didn’t hit it directly. The Americans kept the proximity fuse very hush-hush; if I recall correctly, it was restricted to being used at sea, where unexploded shells couldn’t be recovered by the enemy and studied for their secrets.
Another effective weapon whose details were kept secret from the American public was the shaped-charge warhead. I own a laminated WWII magazine advertisement page which tells Americans about a revolutionary new anti-armour weapon called the bazooka; it makes the point that GIs are finding it very effective against German tanks, but it adds that the exact details of how it works and what it looks like can’t be revealed. As it happens, the Germans knew all about the Munroe Effect (discovered as far back as the 19th century) and had their own shaped-charge anti-tank weapon: the Panzerfaust. An amusing example of the public on your side being kept in the dark more than the military people on the enemy side.