As the maintainer for Windows in-box bsdtar/libarchive, I'd love to learn more!

How are the tarballs you get out of it invalid? Do you have a sample file I could take a look at?

FWIW, since it is bsdtar it supports a few additional options[1] to control the format. You can pass --format=gnutar, --format=zip, --format=ustar, --format=7zip, ... for a bit more flexibility and to create archives in other formats.

[1]: https://man.freebsd.org/cgi/man.cgi?query=bsdtar&sektion=1&f...