Restic’s repository format is simple and well documented, which is important for long term data recovery (and fixes in case changes occur in the repo). The crypto is from a good source, and well regarded. Multithreaded, fast, nice and clean output.
ZFS is a file system, and has serious limitation when used as a backup tool. It needs a ZFS backend, ruling out almost any provider (basically self host your ZFS system, which is costly and error prone). It needs more RAM than Borg and restic. And I personally feel uncomfortable with native encryption in ZFS til some time. Lower level system encryption is probably not what you want in backups.
One feature I miss from these tools (other than ZFS): error correction. They could use a Reed Solomon code or similar and add parities in case there is an accidental change in the repository.