You're speaking of functional completeness of boolean operators which I think is quite unrelated to theories of computation. For computation you need some notio

You're speaking of functional completeness of boolean operators which I think is quite unrelated to theories of computation. For computation you need some notion of state, and while a circuit made of logic gates may possess some notion of state, a logic gate in itself does not. As for transistors being universal (whatever that means) if this is true then vacuum tubes, relays, pneumatic valves, etcetera are also universal.