Have you tested a full book with hundreds of pages on some other device?
I think the disadvantage would be concealed on iPad and other powerful device, but may be apparent on slow devices.
But if you create some ePub brochures from Word, then one file will be fine.
And I agree Word is a very good Editor for writing books.
Here, please permit me to give a slight advertisement for my product,
ePub Maker can optimize Word HTML, get rid of those garbage code, and it can split Word HTML and generate TOC.
You will find it a good tool if you like using Word writing books.