He's a good driver, absolutely no doubt about that, but definitely not the miracle maker he's often seen as. Getting prefential treatment over team mates explains quite a few things and the years at McLaren just about confirm it as his performance there has never been significantly better than that of the other guy. Vandoorne gets a pass as of now, it's his first year after all.
You can still argue that for almost any top driver; Senna got preferential treatment over Berger, Häkkinen got preferential treatment over Coulthard, Villeneuve got preferential treatment over Frentzen, Vettel got preferential treatment over Webber, Schumacher got preferential treatment over Verstappen, Herbert, Irvine and Barrichello. In fact, the times when teams were playing the 50/50 game (publicly, at least) it has frequently lead to friction and fallout; Pironi and Villeneuve, Senna and Prost, Hamilton and Rosberg, Hamilton and Alonso. Even Tambay and Arnoux having a 50/50 team split arguably cost Ferrari the 1983 title.
The thing which separates some of those cases (Villeneuve/Frentzen, Häkkinen/Coulthard, Vettel/Webber) from other cases (Hamilton/Alonso, Tambay/Arnoux, Pironi/Villeneuve) is vindication. Did the ends of winning the title justify the means of favouring one driver at the expense of another? In the former cases it did (1997, 1998-99 and 2010-14) and in the latter cases of not favouring drivers, it didn't (Räikkönen winning without being considered for the title in 2007, Piquet winning in 1983 and Rosberg winning in a tumultuous 1982); Piquet himself benefitting from a strong advantage over his teammates Hector Rebaque and Riccardo Patrese.
Furthermore, when Alonso has had a good car he has crushed the likes of Giancarlo Fisichella and Felipe Massa at times and even with a mediocre car put Nelson Piquet Jr in the shade. His 2005 and 2006 titles vindicate his preferential treatment at that time. His and Hamilton's failure to secure the 2007 title does not vindicate McLaren's attempt at not publicly favouring one driver or another. Alonso isn't a 'miracle maker' though, no. That's fair to say. The McLarens
are terrible with him at the wheel or someone else. But I don't think you could put many of the recent drivers in some of the McLarens he has driven and gotten much more performance out of them.
He's always going to carry clout and influence being one of the top drivers of his era and a double champion but he is also under a lot of scrutiny exactly because of that past success now that he is in bad machinery, whether through his own fault or not, as evidenced by the interest in his career throughout the F1 world to this day.
Sky now think that Kubica is a no-go for Renault in 2018 and they wonder if he might end up at Williams.
I can answer that one: no, he won't.
Why are Renault no longer interested in Kubica?