GnuCash Project Summary
As is typical for GNU projects, this one comes with a large, 18-chapter book. It's big and complicated.
Somebody put a tutorial up on YouTube (which is ironic because GNU activists hate invasive social media). The tutorial doesn't show much--for instance, it doesn't show how to enter data. But it's not a strong advertisement for GnuCash; the speaker doesn't like it much.
There's an externally exposed API, but the wiki says: "Unfortunately there doesn't exist any up-to-date coherent set of documentation. Part of it is in doxygen, but part of it are only in the C header files."
Free software advocate Micky Metts has chosen GnuCash for her company, Agaric, so she must find it useful.
Indications of maturity level
The earliest version on the web site (version 1.6) was released in 2014.