I may be biased but the OpenBSD approach with pledge() and unveil() have been my favorite sandboxing mechanisms of all time due to their simplicity: pledge has really understood that as a developer I want to whitelist an intention, not a specific set of syscalls and options, and unveil is chroot on steroids <3
We will publish a comparison but I'm cautious as it can easily look like an attack over what others do and I feel strongly about not being hostile to other open-source projects :-)
Long story short: we provide multi-source/multi-destination/multi-storage (ie: backup S3 to disk, restore to SFTP), we have a nice UI, we reimplemented our own database over CAS allowing us to have a virtual filesystem + a ton of nice features on top of the snapshots, + an archive format of our own and other nice features.
All of this is in the free version, what's going to be paid is plugins to backup commercial services, enterprise features like multi-user support, ACLs, or compliance related features (ie: GDPR / sensitive data detection, ...), backup orchestration over a pool of machines, and more.
first, we're going to release pre-built binaries for various platforms with our next stable release which will remove the need to install a go runtime as you'll have a standalone native executable for plakar / ptar.
then, the format is open and we'll publish a friendlier documentation should someone want to implement their own builder/reader.
finally, it's likely we'll publish a library + standalone executable in C, so that people can easily implement a binding to their favourite language and/or have a small executable in a language that traverses the decades :-)