I don't hate her because she isn't real, I also don't play anymore, but if I had to answer why I would hate her, and HATE the idea of her being War chief it has more to do with the fact she didn't fucking earn it.
No no, I didn't play Legion or anything after WoD I have followed the lore as best I can, because I always tried to enjoy that, but even what I have seen she hasn't fucking bleed for the Horde, not really, her story is tragic, I think her potential not only as a WoW Icon and a pretty kick ass female to boot, the problem with her is that wrong or right, she hasn't been distinguished enough, nothing but for her own personal story.
That is blizzards fault, she isn't the worst, Auduin is another, cute cinematic, nice, but I wouldn't trust this kid to manage a parade. he is too young, he has a unique perspective, but he has NO TEETH, and No SCARS.
Some critiques of Sylvannas are going to be unfair for lots of reasons and not all are equal, especially when it has to do with who they don't want her to be, rather than the her place in the story, and not being developed enough, maybe Blizzard will turn her up to 11 now she is War chief.
In her defense though, While I can see people agreeing with my opinion keep in mind, it isn't like I have a better Idea for War Chief because I don't there aren't Blizzard needs to do more with the story than say "Buy the book"
No Blizzard get off your ass and bring it to life, people want to see it.