Are you serious.. SAT solvers have some of the hardest code I have seen. Look at the SAT solvers that won satcompetition. On the other hand IMO, implementing a neural network on your own is very easy. But the point is not that. I consider the neural network libraries(e.g. tensorflow) as given, as it is in not related to the problem, and can be used in many different problems. I will consider only the code above the neural network API for the complexity(like models, encoding etc.). But a SAT solver has a specific function and is core part of the problems it is trying to solve.