Fallout technological issues and limitations aside, could synthetics have worked in the Fallout setting if the writing behind them had been better? (it could always have been made up that they originate from the one place were transistors and microchip processors were developed) I am always open for new additional 'species' to the Fallout setting such as the Ghouls, Super Mutants, Ghost People (kind of Ghouls), the planned Troglodytes (not the ones from The Pitt), the Beastlords (I actually kind of like them), and more in such spirit. (I am not sure if we really can mentioned Psykers in this category as they are really really rare and it is not a mutation you can randomly pick up and I am not sure how many people inherited it) So in that context I would have been willing to give Synths a chance. Still I am not sure how far they should have gone at becoming like humans such as the Synth 3s. I would rather have made them a unique 'species' of their own that only emulates humanity to a certain degree but for the rest are more like walking computers. Sort of like if Lt-Cmdr Data from Star Trek TNG decided to create a society like him.