Jamie and Brienne should have stayed platonic and respective relationship, their sex scene was the most forced thing I have seen in this show, made to pander to the fans, it was fecking awful. On the other hand, him going back to Cersei I have no problem with, Jamie was always a gray character, him going beck to her does not erase the good he has done before, I hate people wanting this fairy tale Jamie ending where he becomes this full blown good guy and stay happily ever after with Brienne.
So yeah, all this Jamie character arc ruined I don't agree at all, the only thing his character was ruined with was that awfully forced sex scene.