I call it a docs system rather than static site generator because the web is just one of many output targets it supports.
To tap into its full power you need to author in a markup that predates Markdown called reStructuredText (reST). It's very similar to Markdown (MD) so it's never bothered me, but I know some people get very annoyed at the "uncanny valley" between reST and MD. reST has some very powerful yet simple features; it perplexes me that these aren't adopted in other docs systems. For example, to cross-link you just do :ref:`target` where `target` is an ID for a section. At "compile-time" the ref is replaced with the section title text. If you remove that ID then the build fails. Always accurate internal links, in other words.
The extension system really works and there is quite a large ecosystem of extensions on PyPI for common tasks, such as generating a sitemap.
The documentation for Sphinx is ironically not great; not terrible but not great either. I eventually accomplish whatever I need to do but the sub-optimal docs make the research take a bit longer than it probably has to.
I have been a technical writer for 11 years and have used many SSGs over the years. There's no perfect SSG but Sphinx strikes the best balance between the common tradeoffs.