For a while, I was the only developer working in a small module of a bigger project: I started the code base, discussed requirements with clients, implemented the needed features, tested the whole product end to end. I developed a very good instinct about it and about what any change would do, much better than any other project. My theory is that the code base matched my way of thinking, so thinking about it was pretty easy.

The other thing is style - when used well, state machines don't require testing. There's nothing to test - either your machine works or it doesn't, there is no point testing state transitions because that is the fundamental job of the state machine. You may as well test that addition works.

Ofc, they must be used well - problems may be difficult or overly complex to model as a state machine or even set of state machines - the pattern excels for small problems, less for large ones.