We developed an Entity DBMS where the entities are FSMs.
https://medium.com/@paul_42036/entity-workflows-for-event-dr...
I believe that FSMs are a very powerful approach, even for building entire systems. So much so, that it forms a core part of our product.
Yes, FSMs are underestimated in imperative programming. Of course this is probably because before type-safe state machines were available, manual coding was very error-prone.
I've seen them a lot in embedded programming, but I can't be sure if that observation holds in general or just for the environment in which I was working when I did embedded.