I think we should view the ghosts as a sort of sentient light battery that opens doors/hacks stuff while alerting every enemy in a 3 mile radius of your presence. The battery needs to be recharged by being in the presence of light (near the traveller). The further away they are, the more their activities cost the to the point they run out, which is why that one dead ghost found by a dead orbit expedition died, because it was too far from the light for too long. And being in the presence of the darkness slows it's abilities to revive (some darkness slows it down because it can't gather ambient light fast, a lot stops it because it cannot gather enough to revive on its own at all, and even more means it cannot revive even with help, Aka hard mode). Additionally, the hive have an ability to drain it even more than usual, as evidenced in the dark below grimoire cards where eris' fireteam's ghosts slowly lose their light. And finally, if the battery casing is damaged, it loses energy on its own. So a damaged ghost may hemorrhage some light. A destroyed ghost loses all of it.