The documentation for
polyglossia located on CTAN takes 17 pages, while
babel documentation is (without the desription of the code) about 50 pages. Also there are a lot of documents
babel-<language> to describe some specific stuff about the language.
As pointed out in the answer Russian hyphenation with polyglossia there is at least one undocumented feature.
Are there any other undocumented features or does
polyglossia rely on
babel in some way such as using shortands defined by