Great points all around, hydrospanner.
I am not sure that Kylo is that inconsistent, though. What he has in sheer power and connection to the force is not the same as discipline. This is, of course, the Dark Side. What we’re led to believe is an easier and faster path to raw power. And Finn only probably only lasted as long as he did in the saber battle due to Ren being severely injured by a blaster in a moment when he was surely distracted.
I’m also looking forward to VIII. Rian was apparently on set for a good deal of TFA filming and in direct contact with JJ, and even made some suggestions to make his life easier later. I hope he goes nuts with it in his own way, while keeping these characters as human and enjoyable as JJ/Kasden left them off.