I wrote a ~200 page (digital) textbook on 3d Modeling using Word.
Word is incredibly customizable, I had hotkeys setup to do everything I wanted, figures and diagrams manages themselves rather well, and I was easily able to get into a state of flow using Word.
Now it took several days to get Word that customized, but having seen what it can do, I will not call it mediocre.
However hot Word tip of the century is, and has always been, put all images inside a text box, they'll behave the way you expect then, and behave bizarrely otherwise.
Should be noted I wasn't doing any typesetting or any such thing, just writing the document, I understand that beyond just writing, LaTeX is a far superior tool.