[rleigh]

Let me just throw in:

ETL State Chart and Hierarchial FSM https://www.etlcpp.com/state_chart.html and https://www.etlcpp.com/hfsm.html

Quantum Leaps https://www.state-machine.com

I've used them primarily in safety-critical systems where complexity, timing and the ability to effectively verify behaviour is obviously important. Being able to separate the decision-making from the actions is a great aid. Having to strip back the decision making to "what do I do next" when I'm in this state and this event occurs is a bit different to how most programs are structured, but really does aid separation and makes it easy to reason about behaviour under different conditions.