Simply put, it's designed to make PDF output and provides a lot of quality-of-life tooling around that. Better support for CSS paged media and non-HTML XML input, better handling of page-spanning tables, shims for things missing from paged media (like hyphenation and ligature controls, PDF-specific objects to modify output with Javascript, Javascript embeds into PDF output), better out-of-the-box styles, better documentation and support, and more predictable output and better debugging tools.

Browsers are catching up on paged media options, but they're not all the way there yet. The rest of the stuff Prince does is not (and arguably shouldn't be) in the scope of headless Chrome.

There was a brief moment where headless Chrome supported flexbox in print before Prince did, but that was fixed a month or so ago in Prince dev and should land in production soon if it hasn't already.

PrinceXML is in a league of it's own. Very, very light weight. Very fast and it has outstanding output. The price tag is steep for sure, but if your business relies on PDF output then nothing comes close.

Do you use it in the context of technical documentation? Does it make using Docbook a bearable experience?

It has unmatched print CSS support, so it'll take anything you throw at it.

Also it blisteringly fast. You'll process 10 documents before headless Chrome has even finished starting up.

There's a free trial which prints a watermark so you can give it a spin. very easy to get up and running, even has a GUI so that you can throw some test docs at it without any effort at all.

You should check out their samples: http://www.princexml.com/samples/

Stuff looks like it was created with QuarkXPress or Adobe InDesign.