Sort of.
It's a ZIP containing a collection of XML files. The actual content is a single file, but you need separate ancillary XML files for things like styles, links, headers/footers, numbering schemes and so on. Each with complicated namespacing and nesting rules and each file referencing items in the others.
The area where it is most at risk (for me personally in practice) is the lax handling of the file by Word itself. As a specific example there are nested XML elements where sibling child elements define properties on the main element, such as properties that define paragraph styles.
Being siblings they should be supported in any order, but in practice generating DOCX files and importing them into Word will fail for no obvious reason, until you reorder them in the raw XML (even though they are still at the same level in the hierarchy). Then they work.
In other words, it's less the 'spec' and more the MS implementation that makes it fragile. And different versions of Word can have different behaviour in that regard.
I've used Word professionally since the mid-1990s. I do know how to use it properly, and it still sucks.
"Writing" isn't meant to be done in a word processor, which was developed as a business tool, not a creative tool. Writing should be done in whatever tool one wants to write in.
Word is, be design, both a desktop publishing app and a secretarial tool. For book-length writing, it works poorly with long files, the file format is subject to corruption. The docx format is also proprietary and subject to Microsoft's whim; any conversion scheme is a hack (though Pandoc and many others do work adequately). Unless you learn the ins and outs of Word's style scheme (and sometimes even if you do) and follow it slavishly, formatting is often inconsistent and there's no certainty that the styles you apply to make your document to make it look a certain way ensure it looks that way on someone else's machine.
There's no doubt, though, that a Word-compatible word processor needs to be in every writers' toolkit, since it is the standard in the publishing world.
[0] https://www.sfwriter.com/wordstar.htm, scroll down or search for THE LONG-HAND PAGE METAPHOR.