Nero may not have been a particularly memorable villain, but he was still better than Shinzon. At least Nero's insanity was more apparent and his mad quest for revenge, as problematic as it was, made SOME sense, unlike Shinzon. Spock did promise to save Romulus, and failed, resulting in the death of Nero's pregnant wife and the extinction of most of his race. That may not be much, but it is still more motive than Shinzon, who has NO reason to hate Picard or Earth and EVERY reason to hate the Romulans. Also, trying to justify Shinzon being poorly written by bringing up Nero doesn't change the fact that Shinzon was poorly written, it just means that Nero was as well, but Shinzon was written worse.