Soong wasn't concerned with creating an android, he was concerned with creating a mind, and arguably, a life. The Data-style chassis was secondary and probably trivial for the Federation to manufacture. The biological synths appear to be even closer to a human mind than we saw Data to be in some ways.
Nothing about Data's design is trivial. Sure, the positronic brain was his utmost preoccupation, but that's only because he had so much trouble with it, & faced pretty much everyone telling him it wouldn't work, but everything about Data's design is a fantastic achievement, even more so for Juliana Soong, who was nearly indistinguishable from a human. They all marvel at the complexity of design, that's intention was to create a machine that mimicked our own as much as possible. His goal was to create an artificial humanoid, & yes, the most complex part of that was to create its mindBashir said:Your creator went to a lot of trouble to make you seem human. I find that fascinating
I mean why would he make the thing capable of dreaming? What need does a computer have for dreaming, except for it to live as we do?