What's the problem with event-driven state machines in Haskell that leads to "complexity disasters"? Maybe you could show an example of a non-disastrous imperat

What's the problem with event-driven state machines in Haskell that leads to "complexity disasters"? Maybe you could show an example of a non-disastrous imperative variant, and we'll see if me or someone else can come up with a Haskell translation that isn't disastrously complex?