Do you think Nemesis would have been better if Shinzon had better motives? Shinzon was supposed to be Picard's Khan, but Khan was an established villain who had a personal reason to hate Kirk, Shinzon was some evil clone that came out of the blue and hated Picard for existing, even though he would not exist without Picard and Picard had nothing to do with his misfortunes.
Khan's motives are far more compelling then Shinzon's, to the point that I think Shinzon's motives should have been reworked for the film. Shinzon being an cunning Romulan commander who wants to defeat the Federation, a Romulan counter part to Picard or keep Shinzon the same, but make him threaten Romulus rather then Earth.
Plus Shinzon seems to change characterization through out the film, at some points he is supposed to sympathetic, at other times he is presented as a genocidal mind rapist monster. More consistent characterization would have made him better.