MobX State Tree advocate here. We have a VERY complex state that needs to be synced with the server and persisted locally and nothing fulfilled our needs like MST.
Well, you know how MobX itself is very unopinionated? MST is very structured and opinionated. It's also immutable which means you get snapshots of every mutation to the tree which is very useful in almost all cases.
Also, sign me up to the Sagas-are-bullshit club.