I bet that particles physics would become a problem only because that they would use too much RAM. Like I mentioned before, if each particle used only 1 byte of RAM, then that would solve the problem, but if each particle uses 1 KB or more of RAM than that would be too much for computers to handle if they have 4 GB or less of RAM.
Particle physics as SPH would perhaps bring lots of bugs and break many existing features in Universe Sandbox, making its performance much worse than it is now, so developers would be to be well prepared before SPH works properly before releasing it. In my point of view, SPH is not an important feature, so developers can wait a few years before it is complete and that it works perfectly well in its beta testing phase before being release as an official update for Universe Sandbox.