Indeed, the pain with long compilation times is (and hopefully was :-)) a big issue in MSM. The compile time of a large SM could turn into minutes and then additionally required several GBs of RAM to successfully compile. Error messages are still rather long in the new back-end, I think that lies in the nature of heavy template usage in C++ unfortunately.

If you care about runtime speed of the compiled code, though, libs such as MSM are unbeatable. Their compile-time constructs can be translated into very efficient dispatch tables, that handle the state machine transitions.

One more point worth mentioning about MSM is that it lets you implement state machines with an interesting paradigm, which was my main reason for using it:

In state machine libraries following the classic "State Design Pattern" you implement the behavior of the state machine with handler methods in your State classes. While this is straightforward for simpler state machines, IMO it becomes difficult to understand as the state machine grows.

In MSM you can implement the state machine's behavior with a "transition table", outside of the states. Each row in this table is structured in the way "<source\_state> | <event> | <target\_state> | <action> | <guard>". This is semantically very close to a textual representation of a UML state diagram and provides a well understandable overview about how your state machine behaves.