As said in this answer, the ebook package is convenient in making a document specialized for phone screens (font, position, spacing, etc) and it's easy to turn on or off (need not take care on the geometry and documentclass). I do see that it makes me some inconveniences, (
\mkbibbold doesn't work while
\textbf does, only using xelatex gives the most aesthetic result), but @UlrikeFischer said that
don't use it at all, it seems to have only a vage idea about how to change the font setup and uses a more or less random set of commands
So what are this package's disadvantages? Why is it not good to use?