I think something has got worse recently.
For me (on my desktop Mac) any docx files modified since April this year were not being indexed. What have I discovered (and this content is largely from my post in the discussion referenced by Rumboogy):
1. Microsoft Office.mdimporter is supplied by Apple (not from Microsoft as part of Office as I had assumed) and is regularly changed even though the version number remains at 12.3.0 and the bundle's creation date remains at 27 June 2011. The contents the bundle do change. New versions are certainly included in the 10.12.4 and 10.12.5 updates (I have not checked older updates). And this Apple version is the one in /Library/Spotlight - it does not come (directly) from Microsoft.
2. If you delete the importer that may partially fix indexing of .docx files, but means that Excel files are no longer indexed! I believe this is not a good solution.
3. I have the problem (non indexing of docx) on my desktop Mac, but not on my MacBook. Both are running 10.12.5 and Office 15.34. I have copied Microsoft Office.mdimporter from the MacBook to the desktop. And the problem is fixed!! All new/modified documents are being indexed correctly. As far as I can see the two importers are identical.
4. Doing mdimport -d1 <path to docx/xlsx file> in Terminal still gives an error about 'wrong architecture'. This suggests there is some 32/64 bit issue. But automatic indexing is working.
5. My solution is to replace Microsoft Office.mdimporter with a copy from a Mac which is working correctly. Keeping my fingers crossed that it continues to work for a while. It might work for you if you find another Mac working correctly.
6. I don't understand this behaviour!