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
Vader was a quadruple amputee in his 50’s and had weakened himself to unimaginable extent after he lost to Obi-Wan in Mustafar. So Kylo Ren in terms of raw strength might’ve been in Vader’s league. However, I don’t think Luke had that much time and was enough in tune with the force both times he faced Vader to be able to sense his raw strength. I think he was more focused in Vader’s feelings in RotJ and in ESB he was just shitting his pants, really.
Luke has no way of knowing Vader’s raw strength, meaning before Vader was trained as a Jedi.