I always got the impression that the general hardware wasn't so much a miraculous invention, as it has been seen in everything from the Enterprise, to an Exocomp, but the specifics of creating the Soong Positronic Matrix was the mystery, even to Data, who failed at creating a stable one, & Soong having flaws in all his creations prior to Data & Juliana. Shit, Maddox has made ground breaking cybernetic developments in his work I'm sure. Data finds him a worthy scientist, but even doesn't have a clue how to achieve the matrix. He doesn't even have a good plan for how to study one.
As impressive a design as his body is, the real magic of Soong's work was in the mind of Data