It just never really stuck with me like the other films. By weakest, I don't mean that it's bad. It's still a good film and I enjoy the visual differences between the "real world" 2D animation and the fictional papercraft animation. I think I just find the Tomm Moore directed films to be more visually striking with their emphasis on having flat perspective over the more traditional art that's in Breadwinner.
Visuals have a lot to do with the film for me. Story wise, I think Breadwinner is...fine. It's not really all that fantastical, but it's not supposed to be. That's fine, but when you go from Kells and Song to Breadwinner, the more grounded nature feels a bit off for me.
Again, it's not a bad film, and I'd even recommend someone who hadn't seen it to give it a watch. I'd still recommend Kells or Song first, though.