I don't question his skills in any way, its just he does a lot better when he either has tame to establish a setting or has a fairly rich setting to work with, I don't think these are the only conditions he can work under, but i think thats how he gets the best result.
if you look through his past works he really shines when it comes to group dynamics and characterisation, but both of these need context, avengers was a fantastic film, but alot of the characterisation took part in there own films and Whedon just brought it together (excellently in my opinion) but the film on its own, although very good defiantly benefits from him being able to gloss over the back-stories and go straight to the whole group/fighting thing.
as i have been lead to believe they are producing several starwars films, i think his talens would be better suited for one of the following films and not the one aimed at establishing the film. also my knowledge of star wars is...well lacking, so i cant say how he would do with the setting of the show as he defiantly has his own ideas of how things should be in the things he is involved in, and starwars is an already very well established setting