Most of the time, it's just jealousy.
I've never understood haters. They really must have nothing to do in their free time.
She does have talent; there's no question about that. Her lyrics, her attitude... Her voice is also talented, even if it's not perfect. She's sincere about everything, and has no lies in her work. (quoting Ayu herself here
) She's successful, so people are jealous.
People waste their time on all their hate and don't even give her a chance, really.