Aurora significantly modifies the internals of the DBs, particularly the storage layers. It also makes large changes to how memory is used for Postgres. Query plans can be quite different than with the vanilla version. Once you tune and create indexes based on Aurora's characteristics it's going to be a pain to retune for the unmodified version. Aurora also introduces nasty bugs that don't exist on the RDS version such as a memory leak I found was periodically restarting our master. The Postgres team produces highly reliable code, but I don't trust Aurora's hacks on top of it.