The website is pure slop with an infinitely low information to noise ratio. Judging by their documentation[0] it's just access control for web servers, so the better question would be how it compares to TLS client auth (which uses proper standards, doesn't require users to install extra software, and doesn't require any extra daemons server-side)
[0]: https://docs.opennhp.org/nhp_quick_start/