Based on the comments and remarks of this thread I can make a conclusion on why she is hated but again its just my opinion. Saskue is now one of the most hated character in the whole of naruto series I guess because he is kind of a sociopath that kills for no reason and has poor excuse to annihilate whole of konoha.Being said sakura still has feeling for him even though he is a villain and that dont go well with Naruto fanboys.
Lack of fighting abilities and always being a hindrance due to the fact that fans dont understand the duty of a medical ninja. Her job title says it all but I guess its not enough to be acknowledge by a bunch of Hinata fan boys and girls because they believe she is more useful in fighting and overall a better team player than Sakura. Fighting using powerful jutsus and taijutsu determines how powerful a shinobi is stated by Orichimaru and I believe that most of the Naruto viewers agree with what Orichimaru stated cause they dont understand the value of support and team work that Sakura displays in her battle. People are obsessed with powerful jutsu's including myself that we fail to acknowledge the ninjas that are more of a support type who would even sacrifice to heal the life of another for example tsunade risking her life to safe Naruto and Sakura risking her life to safe chijo bassama who is not even a ninja from the leaf but a mere old acquaintance who she only knew in a couple of hours. The girl ran in front of a sword that pierced a vital organ that is the tensity of how far they would pushed to save someone.
Fans in Naruto base are not mature enough to understand and individual emotions to there love ones and they don't comprehend mistakes that sakura made due to her obsession of saving Saskue. Kishi I must say is very intelligent explaining different type of personalities and elaborating on there strengths and weaknesses of dealing with stress and challenges that can be overbearing for anyone in the case of Naruto and sakura trying to safe a brother who is basically mentally ill. Sakura is more the introvert:meaning( passive,shy,sensitive,easily hurt,moved by emotions) where as Naruto is more the extrovert:strong willed,confident,aggressive,etc). Each character deal with the leaving of saskue differently and I guess people tend to prefer the confident,driven individual (Naruto) than the supporter,shy,emotional(Sakura).
Sakura to me is just a typical female who is in love with a criminal definition(badboy).Feelings dont just fade away and my knowledge of women I can confidently say she is basically playing the part of a girl being desperate to safe her love one. People may not like the way she is handling it but To me she is a reminder of women who would go the extra mile and thats how I understand it. I like sakura persona and to me most girls are like her and she is quite similar to hinata and I just cannot understand how people cannot see the resemblance between Hinata and sakura but to me it illustrated clearly Hinata desperation to safe naruto and sakura desperation to safe saskue they are so alike. Ive posted this already but just needed t do it again. Hinata and sakura are equals in terms of combat and personality stop trying to make them enemies cause they are both beautiful in there own way just saying.