I don't think it's complex but maybe it's unpredictable (or not easily predictable). It's all a matter of trust though, these expert systems are going to have to prove themselves to be superior to human judges (such as in cases of mistrials) before people can trust them.
They don't have to have encountered all possible combinations of facts either, machine learning is good at generalizing. In fact, civil law systems have already codified everything, and the system works. Also having machine-judges doesn't mean we can't add new laws when needed.
This system would lead to a whole class of "artists" trying to game the system by manipulating the data before feeding it to the machine-judge (a combination of what's already happening in courts + SEO tactics).
Edit for your edit: civil law still has to be constantly amended.
The main difference is that civil systems unifies the judicial and legislative functions under the premise that it can all somehow be decided in advance for every case.
Well, to answer your question, humans will come up with these laws (through legislative action or sth, IANAL). An expert system wouldn't need to record its previous decisions since it can come up with them when needed. In this sense it can be more adaptible than common law courts.
They don't have to have encountered all possible combinations of facts either, machine learning is good at generalizing. In fact, civil law systems have already codified everything, and the system works. Also having machine-judges doesn't mean we can't add new laws when needed.
This system would lead to a whole class of "artists" trying to game the system by manipulating the data before feeding it to the machine-judge (a combination of what's already happening in courts + SEO tactics).