[PJDK]

I think you could kind of generalise this to say that a mission statement is only useful if you can imagine some situations where someone would have a difficult call to make and could reference the mission statement to help make the decision.

The one I always think of as good is "Move fast and break things". Not that it necessarily is a good idea, but rather that you can quite clearly use it to help make decisions like "do we launch now to capture a transient opportunity or later to ensure full QA?"