I don't think anything is "wrong" with Backbone. It's dated in its ideologies though. Backbone comes from an era of script tags and a global "Backbone" namespace. Until recently, it required jQuery to do its "innerHtml/append" DOM manipulation. As a way to add structure to a jQuery app, it's great. It leaves a lot of freedom to the implementer. I did find that I required something like Marionette to assist with views however. Many developers are starting to stray from the MVC pattern toward other (perhaps more interesting) patterns and I think this is where some of the koolaid-style "hate" comes in.

Nothing is wrong with Backbone. I thought it was beautiful. We don't always pick our tech though. It's part of the problem here; the hype engine is used to steer non-technical people, or more managerial technical people. While the engineers and technical leads might be completely comfortable; the next project starts and you are told it should use X since that is what the client kept mentioning when they called.

Also, some people love this. It means they can show up at work and talk about how they learned React and it's better and they can do a pretty good job of using the hype train to stand out from their peers.