The article is probably underselling general rel here. The Einstein equation is famous for packing a whole lot of 4-dimensional, nonlinear tensor nastiness int

The article is probably underselling general rel here. The Einstein equation is famous for packing a whole lot of 4-dimensional, nonlinear tensor nastiness into an elegant little package. You need to add something like an equation of state to turn it into a complete dynamical system, but I am suspect that there is enough nastiness in the gravity part alone that it will be difficult to solve in many regimes. What saves us is that we are usually concerned with low-temperature, low density systems, or else highly symmetric like stars and black holes.