Shinzon didn't irrationally go after Earth. The first scene in the movie establishes that's what Shinzon's motive was. Destroy Earth = Crippling the Federation. It's repeated throughout the film. Shinzon says this to Picard, Picard and Riker both deduce the reason why Shinzon would attack Earth.
I know it seems more logical to attack Romulus because he hates them, but Shinzon has intergalactic aspirations and he can't do that without support from the Romulans.
Star Trek XI's saving grace is the spectacle of the film. The film's big budget makes the world JJ created look incredible. XI is 80% style and 20% substance, the bulk of that substance comes from copying and pasting successful ideas from the previous Trek media.