My point about an out of date sat nav being dangerous is that if one relies on it (as many do) you can run into big trouble. But of course one should reference your navigation to an up-to-date map. Correct me if I am wrong but it is a legal mandatory requirement to carry one.
My main point, however is that if you hire something you enter in to a contract with the owner who has a duty to supply what he states you are getting.
If he informs you that the sat nav is out of date, that is ok.
However, many of us do not update our Sat Navs every month as that is not cost effective for single owners. But then we are aware that errors may be inherent.