Yes, it is a moving target, but you can still get it to a very solid good level. In the past it's always been a few years between updates, and they have version details in the files, so you can say "Supports Word 2007 files" and not worry about Word 2016.

I've written a bunch of Perl code (that I should open source.. maybe some day. anyone here'd find that useful?) for creating and editing docx files. It's not that hard -- it's all XML!

EDIT: I was using a LibreOffice daemon before, but I did this code just because Libre had so many bugs / missing features when converting a document to docx.