Weasyprint has a bunch of options to do almost anything you can think of, but in my case, it's just a generator.
To your point that htmldocs supports more modern CSS features, I can see the advantage. Although the most complicated things I needed - aside from @page rules - were a replacement for a 2nd page header (solved w/ Flexbox) and automatic page breaks that would display correctly (a few lines of CSS processed w/ Weasyprint came out better and didn't require me to print to PDF manually.)
To your point that htmldocs supports more modern CSS features, I can see the advantage. Although the most complicated things I needed - aside from @page rules - were a replacement for a 2nd page header (solved w/ Flexbox) and automatic page breaks that would display correctly (a few lines of CSS processed w/ Weasyprint came out better and didn't require me to print to PDF manually.)