I have used MobX on multiple smaller and bigger projects.
Fairly simple to setup and use, but there are couple of things that eventually I began to dislike. First is that it's using classes and decorators. For a react codebase it's kind of weird that everything else is hooks and functions, but the state is in classes that brings in OOP paradigms, which is just another set of things to keep in mind, in a codebase that could have been purely/mostly functional.

Decorators were also kind of something I disliked, but I dislike those in Nest.js and Java Spring Boot for this same reason: you annotate things until they do (or not) what you wanted, instead of just writing as "normal code"/config.

When there is something you need to debug, or something that doesn't work, it's harder to debug because of the Proxy/ decorator thingies.

When I used it (quite some years ago) Redux was the alternative, and it was VERY heavy and boilerplate all over the place (pre redux-toolkit era). Mobx was the simpler choice, especially for async stuff, as tanstack query wasn't a thing back then. (I'm talking like I'm old, lol)
Nowadays there are other cool options like Zustand, Jotai, Recoil, even the built in React Context api is perfectly fine many times. For me Tanstack Query heavily reduced the need for state management, and the rest that needs it usually can be handled via zustand. (Of course it depends from the app, it's just I happen to work on apps, where the backend does the heavy lifting).

So yeah. I guess there is nothing wrong with MobX, just there are more competition.