"On initial release, the book reader is fully functional but is missing some more advanced features from the main calibre viewer, such as popup footnotes, bookmarks and annotations in general. These will be added in due course. In fact, the browser reader is designed to eventually replace the main viewer, once it matures."
On mobile I would rather use something that doesn't stop working when I don't have internet access. I kind of expect any browser based reader to be thoroughly mediocre compared to actual clients.
"On initial release, the book reader is fully functional but is missing some more advanced features from the main calibre viewer, such as popup footnotes, bookmarks and annotations in general. These will be added in due course. In fact, the browser reader is designed to eventually replace the main viewer, once it matures."