From the Peek website:
Due to system limitations, Peek is unable to work with plain text documents (.txt ,.text ), rich text (.rtf , .rtfd ), HTML/XML/web archives (.html , .xml*** ,.webarchive ), TypeScript (.ts ), Adobe suite (.pdf , .eps*** ), Microsoft Office (.csv ,.doc , .docx , .xls , .xlsx , .ppt , .pptx ), OpenDocument suite (.odt , .ods , .odp ), nor any Apple iWork documents (.pages , .numbers , .key ).
So basically 90% of the files any normal user would want to use Quick Look for 
I do miss the ability to see EPS files in Quick Look. Since Apple sunset the EPS format support, we’ll likely never see it work again.