The only difference between being second driver at Mclaren and being second driver at Ferrari is that Ferrari will verbally announce that they think you're a terrible driver and tell you to get out of the way while Mclaren will keep their opinion to themselves and let everyone else work that out. Either way Sergio is being thrown into the deep end. He doesn't really have any areas where he is better than Button. They both have famously smooth drving styles, they both sometimes struggle in qualifying, and they both are good at managing their tyres. His strong points and his weak points which he demonstrated compared to his team-mate will effectively be cancelled out.