Without looking at the Sylve docs, I'll conjecture that it has deeper integration with ZFS. With a foundation on FreeBSD, there is a likelihood Sylve can support ZFS-on-root rollbacks better than hacking it into Proxmox. A rollback capability is why I'm looking for Proxmox alternatives. In the Linux world, Talos Linux and IncusOS provide A/B updates which achieve a similar rollback capability. With something based on FreeBSD, your "immutable" OS and all of it's data can be treated equally as ZFS datasets. There's also a higher risk that a Linux kernel update will break ZFS.