Scarriest thing, really?!!! Scarrier than facing Darth Vader, the man who murdered your father and your mentor?
Apparently so according to Luke himself:
“I’ve seen this raw strength only once before. It didn’t scare me enough then. It does now.”
In that dialog he is talking about Ben, so I guess he sees the potential in Ben to be even worse that Darth Vader or the Emperor
I disagree. Luke never saw anyone’s raw strength before he started training students, since both Vader and the Emperor had already mastered the Force. So, his only source of reference are his other students, and Ben in the case of Rey, who obviously didn’t turn out that well.