Its a good and common question. Backbone can handle rectangular models okay. When you get into more complicated UI states, or if you have two different views backed by the same ui state like here [1], backbone models totally fall apart - backbone model is designed for list-of-records where the list can change or a record can change, it can't elegantly handle other shapes, like nested records, or list-of-lists, or tree-like (recursive comments), whatever arbitrary shape most naturally models the domain.

Backbone models are also mutable which means lots of model change listeners (callback soup) - check out the flame charts here [2] to get an intuition for what callback soup does to your code and how react fixes it.

[1] http://wingspan.github.io/wingspan-forms/examples/faceted-se... (json blob is the ui state) [2] http://swannodette.github.io/2013/12/17/the-future-of-javasc...