This is an answer to dgbmunger (Nov 30 2021) who wrote "I'd like some evidence that for a 300 page scholarly book Mellel is better than Pages. (…) I'd really appreciate someone taking the time to post specifics."
(1) Mellel has a better outline, i.e. table of contents pane. Bookmarks and auto-titles can be shown together in this pane; thus you can see at a glance where exactly a certain bookmark is in the document. You can also assign "Remarks" and tags to bookmarks and auto-titles and then filter the outline pane based on your remarks and/or tags.
(2) Supports live bibliography with Bookends. (3) Can have more than one type of footnotes. This feature is called multiple Note Streams. One footnote can be added by the author, for example, another by the translator, etc. — (4) For those who write novels and stories a so called "story points" were developed to help keeping track of the plot and its progress. Thus you can quickly see where characters and locations appear in the document.
Pages is regularly updated, and each update has new features. Mellel's development is extremely slow. It still needs elementary word processing features, such as split view, macros, non-contiguous text selection, shapes, attached captions for images, just to name few. Mellel has been surpassed by Nisus Writer Pro which now has much more features: it's scriptable, has macros and two types of inbuilt text expanding features, supports more languages, has much better Find and Replace feature, allows you to create concordances and various types of word lists; you can search simultaneously for text in multiple languages, it has support for South Asian languages (which Mellel doesn't have) and it has a powerful Document Manager. Nisus has also some special features which make the daily writing process easier to handle. For example, the text expanding feature (called 'glossaries' in Nisus) and Multi-key keyboard shortcuts which allows you to press the command key and then any number of keys for each shortcut. For example, you could press the Command key and then type 'cal' in order to automatically insert a calendar. This makes it easy to remember your shortcuts. Mellel doesn't allow you to do this.
I recommend you rather try Nisus than Mellel. If you chose Mellel, be aware that the additional programs you need to buy in order to be able to write like a professional cost extra money (text expanding software like Typinator $29.00 and OmniGraffle for drawing $124.99). Nisus Writer Pro has already integrated these features.