also, both have a weakly motivated central villain whose resentment at one of the heroes makes no sense whatsoever.
At least with Nero you can make the excuse that he was grief-stricken and then spent 25 years rotting in a prison. His sanity may be impaired.
Shinzon is just whiney.
While Shinzon did come across as whiney, his sanity was definitely impared, too. He was raised in a mine, and abused by Romulans since he was a child. How would that NOT fuck someone up severely, even Picard?