> [...] I found the documentation to be sparse and the software ultimately non-functional.
Yes, my first take on the documentation was the same - existing but hard to understand in first proof-read. As I implementing my own sync and token server I really often I was catching myself "hey, you don't have to look into Python code, they really wrote how it should work in docs", especially [0] and [1] pages. There are also few other websites owned by Mozilla, which are very outdated, so also misleading.
Ah, worth noting is about:sync extension [2] and logs stored in the profile directory, which may help you to investigate some issues that might come during development and maintenance.
> Has anyone had any positive experiences with self-hosting the Firefox Sync Server?
I am biased a bit, because I had one very negative experience related to token verification error, which costed me ~4 dollars, before I noticed and I had put service into downtime for some months. However, I am really happy about a whole project. Operational costa are is about few cents per month. It also helped me to write some tools I use on daily basis and preserve knowledge I have learned about serverless applications on AWS.
[0]: https://mozilla-services.readthedocs.io/en/latest/storage/ap...
[1]: https://mozilla.github.io/application-services/docs/sync/faq...