Hard question to answer, as so far I am quite amazed in which kind of applications it has been used for which I did never considered it, like webGL / canvas based stuff and such. But I think MobX shines when your domain model is highly interwoven with lot of CRUD like operations.
It makes less sense to MobX with systems that have a strong focus on event based, append-only mechanisms. This kind of applications are better suited by an immutability based approach. But I think that is a minority of the apps, most applications are about data entry and modification.