The thing that I like about Vader is that he begins the OT as the Archetypal Evil Henchman, and his mask is slowly lowered throughout the trilogy to reveal the conflicted character beneath. Most of the characters in Star Wars follow this pattern - they are established as strong archetypes, only to be revealed as a complexity or subversion of the archetype.

The problem with Kylo, then, is that he’s not really archetypal. He is not fully evil, he’s too tempestuous to rule yet too rebellious to be a henchman, too serious to be a comic villain yet too infantile to be menacing…

In short, he’s a walking contradiction at odds with the spirit of Star Wars. Only time will tell if it was a good move for the series or not, but I understand why people have a problem with him.