if you are to describe something (a quality) then it's mostly in front of the noun
postposition is when there's a new distinct notion (name) created with the adjective added
(often the case in science) - biały orzeł is different to orzeł biały
Biały orzeł is simply some white eagle.
Orzeł biały - is the (official) name of Polish national symbol.
Przedni orzeł - would be some excellent eagle
Orzeł przedni - is a name of a species (as in Latin Aquila sth ;)
pyszny obiad - delicious lunch
and obiad pyszny would make sense only if it were obiad 'Pyszny' - a name of some specific course (or lunch set) (there is no such thing to my knowlegde tho)
zjedonoczone stany are some states that are united (especially not considered as some specific entity)
Stany Zjednoczone - is an entity (isn't it)
tho there were some other Stany Zjednoczone before (Estados Unidos do Brasil for instance) - but then these were also entities
Stany Zjednoczone Brazylii - name of a federal state
zjednoczone stany Brazylii - those states of Brasil that are united (when there are some out of the agreement)