For example an ecommerce order tracking system with different status (in cart, paid, shipped, returned, so on). Spree solves it very elegantly, look at those "events", "transitions".

https://github.com/spree/spree/blob/4687e608b49236c2850500b0...

Also, Spree has this nice intro to FSMs

https://github.com/Humane-Documentation/Spree/blob/master/re...

I also happened to work in a very huge C++ project for the business logic component of a call center, using CORBA and the Reactor pattern from ACE to integrate different parts of the user / ACD / 1st agent / specialist / CTI combo.

But it was a telco project, so the "let's use a FSM" was well thought from the beginning. Turns out you could easily know what the system would do just by reading a huge Smartdraw document, and this also allowed for reasoning on new use case scenarios. It did not matter that the main file was a horrendous 40kLOC C++ class because the Smartdraw diagram was the documentation and everything else was just trivial code.

Actor pattern (akka) is the distributed FSM

Not really, an actor can implement an FSM but it also can have an infinite number of states.

every non deterministic SM can be converted into FSM: "Using the subset construction algorithm, each NFA can be translated to an equivalent DFA" https://en.wikipedia.org/wiki/Nondeterministic_finite_automa...

How is that relevant to the parent's point? Non-determinism has no relation to a system with infinite states, and the NFSM -> FSM construction algorithm provides no help in that case.