The metadata retrieval pairs with a DOI or ISBN [1]. Articles without such handles might not work, nor will articles which are not OCR'ed (obviously) but this can be easily fixed with OCR scanning software.
The metadata retrieval process is a very handy feature, especially paired with Zotero's web browser plugin.
Zotero also keeps a repository of all the citation styles, which are curated and administered by the Zotero team and stored in a Github repo [2].