The developers seem interested in incorporating some GR concepts, like gravity propagating at the speed of light, spinning black holes and ways to better approximate GR. Each week Marsh Davies orbits the supermassive blackhole that is Early Access and comes back with any stories he can find or gets shredded to subatomic spaghetti as he tumbles towards a point of infinite mass. Still, Newtonian physics can get the job done for the most part. Accurate general relativity simulations require supercomputers. They would start with some initial data of the shape of spacetime and then see how it evolves according to the Einstein equations, which are 10 highly non-linear partial differential equations. Instead of simulating N number of bodies, they would be simulating a huge number of points. That is, taking your simulation space, discretizing it to a hi-res 3-D grid and checking the effect that each and every point in that grid has on all neighboring points at every timestep.

General relativity requires simulating the spacetime itself. Newtonian physics just requires n-body mechanics, so it is much easier to implement. However, Universe Sandbox 2 uses 300-year old Newtonian physics, rather than general relativity. If you want to simulate day-long events, like planets crashing, it might be better. Means that if you want to simulate the Solar System over a long period of time, it won't be very accurate. The slower the time, the more accurate the calculations.

Its accuracy degrades as you increase the flow of time. Not sure if these kinds of questions are allowed here, but I may as well answer it.

Universe sandbox vs universe sandbox 2