I would say some of it might be purists that can't stand a new character being added to the story, but if they have a problem with her then they must hate the whole thing in general (in which case, why single her out?) because these movies have their fair share of differences. Maybe some of it is people thinking she's a Mary Sue (That gets thrown around a lot in fandom, often incorrectly), finding her inclusion forced or not liking her because she comes between a ship.

At this point I just think she's incredibly hot and looks like she can take care of herself so I'm fully expecting to harbor a huge crush on her.