the cave held not only vader as something to fear, but the wholly dangerous possibility that luke would become vader.

what that means for rey is interesting, though. she says that when she was in the cave she wasn't scared, even though she thought she should be. whereas luke was clearly afraid in his cave and brought in his lightsaber intentionally.

rey was fearless going in and saw only herself, which i think means she is much clearer about herself than luke was at the time.