A lot of experience of a book off bubblin depends on what code is written under it and how the author wants to present it. Inverting light/black, zoom, choosing fonts are all easy things to implement here, and those are some of the flexibilities the author has when choosing something for their book on bubblin.

Reflow is altogether another beast - I guess it is more useful for books with just plain text and images (lifeless). We're not focused on those type of books.

Also for some books reflow is simply a bad deal. For example the more technical ones suffer badly in the current fragmented ecosystem of book providers. I personally dislike it when our inability to scale fonts turns a 50 page book into a 100 page one for no fault of the writer or the reader.

There is much less control on the design aspect of content within a book and even lesser control on the size of the downloaded files there.

And btw, while we are at it, try the scaling potential of Viewport Width units for book. That does it here for my book!