I think Kylo's interesting, less because of his power or threat, but more because of his emotional instability and the struggle that seems to be constant with him. For me at least, I love the notion that being evil doesn't necessarily make him happy and that he's actually quite miserable doing what he's doing. He's ultimately a very self-destructive person and I think the danger he brings is not knowing what damage he's going to cause to others as he destroys himself.
And I wouldn't be surprised if the Knights of Ren don't exactly fall in line behind such a visibly troubled leader. Hux knows he's underpowered compared the Force users, so if he could get the Knights to turn against Kylo...