I deal in a lot of NOS stuff for motorcycles, and my experience is this:
If it is 30, 40, 50 ormore years old, it may or may not be any good, just because its new. Hard parts are usually fine, but rubber, leather, or plastic is a gamble.
Oft times, a 40 year old design wasn't very good then, and certainly isn't good now. If there is an improved, and available design, you're better off.
Where and how an NOS part was stored makes a big difference. A wet, dark basment may not always be the best place for delicate parts, and by the same token a super-dry hot attic may not be either.
What someone advertises as an NOS part, like Tomahawk mentioned, may not actually be. I have pulled many new boxes out to find an old part in them, or a new part that has deteriorated significantly and is far from new.
Of course with all this, there often times isn't any choice other than using what you can find, simply because there is no alternative.
"Free advice is worth what you pay for it."™