As I recall one of the benefits of spoken Navajo was the difficulty of distinguishing it from a bad radio signal of some other language.
They could also have “fun” and make it very hard to crack. For example they could call a submarine a “metal fish” one day and a “sea snake” the next day and leave everybody scratching their heads. As a native speaker they could use subtle context to convey the message and that turns out to be extremely difficult for an outsider to grasp.