Shinzon didn't unload on the Enterprise because he needed Picard alive. He had no such concerns for the Romulan ships.
Sometimes you have to take story factors into the equation.
But he didn't want Picard alive towards the end of the story. He just wanted to kill everything on Earth.
And he then pounded the Enterprise
to the point the self-destruct didn't work and if he hadn't wanted to go out in a blaze of glory, he would've taken her down quite easily.
Once you have a fish on the hook you don't play around, you reel it in.