My main gripe is that you might have to retroactively change your bloodline if you choose to multiclass into Bloodrager, but chose a sorcerer bloodline that the Bloodrager doesn't have access to. (This can be done in a whole lot of ways, Eldritch Heritage being the simplest.)
I wouldn't mind this so much if Paizo published a bloodline for everything they've made so far, so I can have Android Nanite Bloodrager NPCs ready for Numerian adventures. What's even worse is that a lot of the already existing bloodlines (Like Nanite, or Ork) work really well with Bloodrager already, but we're limited to some of the more generic
But anywho, I like Elemental more than I like Infernal or Dragon, since I think the ability of elemental damage to bypass resistance all the time is pretty huge compared to the conditional instances for the other two lines.