She's not against us, she's just not as much for us as someone like Bernie or Warren.
Her record as a prosecutor is pretty bad, but more importantly, she's a corporate darling. Warner Media is her biggest campaign contributor. That explains why every CNN panel, and every other mainstream outlet, is already declaring her the best candidate for 2020.
Doesn't mean she's a bad person, but she'll inevitably be beholden to the corporate interests that propped her up.
That's how we end up with KamalaCare instead of universe healthcare. That's how we end up with $10 national minimum wage instead of $15. Or how we end up with tuition free college only for people who meet certain criteria. Or how we end up with a war in Iran, etc etc.
Harris, while generally well-intentioned, will meet the corporate ruling class halfway.
Meanwhile, a more progressive candidate would fight uncompromisingly for whatever is best for the people.