In my view, the concept of a finite state machine is a simple one with essentially no learning curve. It is a straightforward concept. I would never hire anyone that described experiencing a learning curve for this concept.
The idea of "when an event occurs, the state changes" might be a natural one, but there's many things about FSMs that are somewhat of a learning curve: self-transitions, sink states, Mealy vs. Moore machines (and their equivalence), non-deterministic FSMs (and converting to deterministic FSMs), optimization algorithms, path-finding algorithms, implementation (imperative, functional, regex-based, etc.)