Word is an absolute disaster for large documents
Typically, larger documents combine material from many diverse sources, and if there is one thing that is /guaranteed/ to bring Word down, it's format fragments copied in during cut & paste. In a larger document, the chances of your file becoming unreadable by Word (by the many format conflicts) thus increase the larger the file gets. Add to that the manual formatting that beginners use in Word instead of using styles and it becomes worse and worse until it eventually can no longer cope and just crash when you try to open the file.
Let me translate that for you: the closer you get to your deadline, the more likely is it that Word will crash and make you miss it. I have rescued many, many deadlines by taking Word files (in .doc) into OpenOffice or LibreOffice which will still open them, clean out ALL formatting and start again by only applying styles. After that, save as .doc and Word would again happily work with the file.
I haven't done this with a .docx yet, because I switched fulltime to using Open/LibreOffice before MS came out with that file format. I only have one copy of MS Office installed somewhere for compatibility reasons, but we're pretty much all using LibreOffice now. It's cool that it works on any platform, and people can work at home with the same software.