Let's say you have a component deep in the hierarchy that needs to update based on a small but frequent change in a large object (thousands of keys). With Redux, you have to figure out how to diff the old and new copies of that object to figure out what changed. With MobX, you just observe the value of the exact key you're interested in.

That's just one example, but I generally spent a ton of time writing complex shouldComponentUpdate functions and therefore making lots of mistakes with Redux. I've found MobX much more suited to building complex UIs with deep hierarchies and tens to hundreds of total elements on the screen at once, where updating only exactly when necessary is critical.

I use MobX for my game Falcross, which I wrote about in the State of React Native 2018 thread from a few days ago: https://news.ycombinator.com/item?id=17316877

I initially used Redux, and I found it actually technically impossible to get the game to perform well using Redux. I tried everything.