See also this answer
At Mendix we have a really complicated model (500+ classes, thousands of attributes, lots of inter data relationships). We have partners that write plugins that modify, analyze or generates these models. Independently we have partners that write visualizations. So we needed a system that was
1) simple to write (partners are often no full time JS devs)
2) highly performant
3) ensures consistency without needing to state data needs / force UI updates
4) CRUD actions and relations are very easy in MobX (no normalization needs etc)
I think the benefit of using MobX is that you have to learn way less concepts and it keeps your code more straight-forward. Since it is unopinionated about how you structure your data, it is more suitable to highly decouple state from components. In Redux your component structure and state structure are highly related each other, especially when performance matters. In MobX this is a lot less the case.