For me, the biggest problem with NEM is that Shinzon's motivations are cloudy. We're told he self-identifies as a Reman and is a Reman nationalist... but out of nowhere, we discover that he hates Picard and wants to destroy Earth.
I mean, I guess I can understand the idea that he'd irrationally come to hate Picard. But the idea that he'd turn his genocidal rage against Earth instead of Romulus as the end of his life approaches just doesn't make sense. Romulus has been oppressing and terrorizing the Remans for centuries; as a Reman nationalist, Shinzon's goal should be to destroy all life on Romulus before he dies.
And the purple rubber outfit. Seriously, whose idea was that?
I was right there with you man. Shinzon's motivations should've been the genocidal destruction of Romulus based on his background. Him hating Picard and wanting to destroy Earth makes sense though. It's not about justice or revenge for the Remans. It was about Shinzon destroying everything Picard stood for. Shinzon forging his own legacy and having Picard's erased along with Earth. Shinzon says this in the movie.
He didn't attack the Romulans because they supported his coup and he needed their support to fight the Federation. In the ENT episode "The Aenar" the Admiral Valdore said the Romulan goal was unlimited expansion and that conquest is the best goal for Romulans. With that mindset the Romulans in NEM supporting a war with the Federation isn't unlikely. Gain control of the entire Alpha and Beta Quads with the potential for a Gamma Quad invasion.