The biggest problem is that words mean things. Nemesis has a definition and it isn't just "evil counterpart". Nemesis is an avenging spirit sent to punish people for the sin of hubris. At no point, even with his pontificiation of all things humanity has become, does Picard actually get so arrogant that the gods, such as they are, decide to slap him down. And Shinzon just doesn't do anything besides say "you had it good while I suffered. I'm going to destroy Earth. That'll show you.
Shinzon wasn't Picard's
nemesis so much as he was the Romulans'
nemesis. They needed to play up the "overthrow of the Romulan government" angle and downplay the "Picard vs Evil Picard" angle.
Instead of Shinzon being a slave in a mine, (which by the way was incongruous with his also being a great general) who leads a Spartacus-like revolt he needed to be written as a Patton-esque "old warrior" who just couldn't handle the newer, friendlier relationship between the Empire and the Federation.
This could have played out in a story with elements of ST III and VI, both of which had similar plot tropes.
The Overlord wrote:
Why not do that from the start and forget about destroying the Federation? There also no evidence he was planning that, so this just kinda guess work. That would have been nice to show, because him acting in Romulan Empire's interests didn't make sense.
Anyway you slice it, his motives for destroying the Federation were extremely vague and convoluted. He had motives for hating the Romulans, he even had motives for wanting Picard, but destroying the Federation, that made no sense.
It makes sense from the standpoint of the "nature vs nurture" subtext of the film. Handled correctly, it would have been interesting to see Shinzon played as "more Romulan than the Romulans". Tie that to the idea I mentioned earlier about Shinzon as Patton/the perfect warrior who can't survive the peace and he would have been a powerful "opposite" of Picard the diplomat and peace supporter.