I don't agree with this with Rey, I felt her backstory made us care about her(I think Daisy Ridley is not the strongest actress though, or at least not directed properly, she doesnt make you WANT to pull for her the same way Carrie Fisher or even Felicity Jones did with their characters) but I do agree for Kylo Ren, not enough backstory. He isn't Darth Vader, a faceless scary monster that we had no full understanding of why he went dark until prequals. We saw his face, his emotions, knew his family, but I felt they never really fleshed out why he went so bad, so he stayed kind of one dimensional. The movie did it's best to wrap everything up, but it was hard.
My biggest quibbles are how goofy this hidden fleet was. Where did all the materials and ship builders come from to keep it hidden? They all have star killer cannons? Where did they find enough soldiers to fly them?