"Therefore, the Python maintainers reserve the right to modify
the marshal format in backward incompatible ways"

sure, don't expect such things to survive version changes.  (Actually, they have been hitherto, and my version "3" I actually changed, to be so, the initial draft being unable to read version 2 data)
But for sending stuff over the wire, caching on disk, etc, its perfectly safe and super fast.  And what is more, the python developers (that's you and me) are super careful to never _crash_ the interpreter, no matter how broken any input data is.