Experiences of someone who's been frequently playing with huge things.
-When you're putting too many objects more than 100km from mission center or if your ship is more than 100km from mission center the skybox starts to shake.
-Models larger than 100km have rendering issues (according to The_E it's a physics problem). Textures start to flicker. INFR1 for example had a real-sized 3D model of Earth for "Nemesis", which worked fine in Retail but looks broken in FSO.
-Gravity can be scripted, there's atleast 1 working script.
These two issues may need to be addressed. Particularly the number of stars and the FS1-style nebulae. And the draw distance. Geometry instancing, occlusion culling and image-based rendering, where 2D elements are manipulated to appear in a 3D world.
For image-based rendering, in the context of a 3D game, it delivers a significant performance boost by replacing 3D geometry that is far enough away for the transition to be imperceptible with a 2D image. Implementation of the technology in this role can be found applied to Rome: Total War's soldiers, Far Cry's forests, and various objects in Guild Wars' environments, such as buildings and flora. FSO can theoretically implement this to faraway ships and objects.
FWIW, many modern game engines as of now can render large distances. FSO needs to address these issues based on experiences of someone playing with huge things as Nightmare provided.