I'm not aware of any documentation which lists this so I used ChatGPT to help me compile a quick list. I took a quick look and it looks accurate, but of course there might be inaccuracies.
1. E-Book Editing – Calibre-Web does not include the advanced eBook editing tools available in Calibre, such as the eBook editor for EPUB and AZW3 formats.
2. E-Book Conversion – The ability to convert between different eBook formats (e.g., EPUB to MOBI) is missing.
3. Metadata Download & Management – While you can edit metadata in Calibre-Web, it lacks Calibre’s ability to fetch metadata and covers from online sources automatically.
4. Advanced Plugin Support – The desktop version supports third-party plugins that extend its functionality (e.g., DeDRM tools, additional format conversions, etc.), which are not available in Calibre-Web.
5. News Fetching – Calibre can download and convert news feeds (RSS) into eBooks automatically, a feature missing in Calibre-Web.
6. Full Device Integration – The desktop app allows direct connection and syncing with eReaders (Kindle, Kobo, etc.), while Calibre-Web is limited to wireless downloads.
7. Bulk Import & Export – You cannot bulk import/export large numbers of books via Calibre-Web as efficiently as with the desktop version.
8. Virtual Libraries & Custom Columns – While basic library browsing is supported, advanced sorting, searching, and virtual library features are more limited in Calibre-Web.
9. DRM Management – If you use Calibre with DeDRM plugins, these do not work in Calibre-Web.
10. Built-in Viewer for Some Formats – Calibre-Web supports reading EPUB and PDF files in the browser but lacks the extensive format support of Calibre’s built-in viewer (which can handle MOBI, AZW3, CBZ, CBR, etc.).