It depends on what level of detail you want the simulation to have. Boolean logic is easy. Analog circuits operating in certain (very limited) domains are straightforward. But - noting that what follows is well out of the realm of my experience - in other domains you have to account for things like RF characteristics and quantum effects in the semiconductors.

Imagine trying to simulate something that used water in tubes to implement logic, except it's analog logic and there's resonance and some of the joints start leaking when they exceed a certain pressure and etc.