Sith ghosts aren't the same as jedi ghosts. Jedi ghosts are basically an ascension into omnipresence, near-omniscience and absolute freedom for all eternity in both the living world and realm of the Force. A sith ghost is more like a traditional ghost. They are bound to one place in the realm of the living, have little control over their existence, may not even get to have a visible form, and unlike other souls, a Sith's spirit is doomed to decay and complete destruction within Illathurion unless they abandon their materialistic ways and seek to restore their own balance. Best thing they can do is posses victims temporarily unless they mastered the transfer essence technique.