Except in the case of Joined Trills the new body they move to has a living conscious person and their mind in there. From what I understand NuData's body was mindless at the time.
Define "mindless." As the book itself points out, and as I've been pointing out in post after post already, a mind isn't a monolithic thing. On the one hand there are the memories, and on the other hand there is the substrate that contains them, the hardware and firmware. Data's memories are not "his mind." They're one component of his mind; the physical brain that contained them was the other. It's not like pouring water into a glass. The brain itself helped shape his behavior, thought patterns, and personality. It's not just an empty vessel.
So what we have here is a brain that's hardwired to think and behave like Noonien Soong, but that's forgotten all of Soong's personal memories and had them replaced with Data's. But because it's still Soong's brain structurally, it should have Soong's ingrained patterns of thought. And combining those with the memories and knowledge of Data should produce a new synergy.
Granted, previous Trek hasn't exactly provided a lot of foundation for this idea. Usually it treats mind transfers as if it were just pouring water into a glass, as if the physical mind itself contributed nothing to the personality or identity but were just an empty vessel for it. Which is completely ridiculous and unrealistic, but unfortunately a pretty standardized depiction. Let's see, we've got "Return to Tomorrow," "Turnabout Intruder," "The Schizoid Man," "The Passenger," "Body and Soul"... as a rule, the assumption seems to be that the transferred personality is essentially unaffected by the substrate it occupies. Although "Turnabout" did imply that the subjects' brains were rejecting the personalities imprinted on them, and "Schizoid" did seem to suggest that Graves hadn't completely replaced Data's consciousness so much as suppressed it.
But my reading of The Persistence of Memory
tells me that Dave is taking a different, more credible approach and acknowledging that Data 2.0 is an amalgam of Data's memories and the Soong android's ingrained personality traits.