Repart so far is best for appliance images, modern uefi/gpt stuff, single-disk, systemd-sysupdate a/b partitioning, and now I think erofs/squashfs has highlighted another key difference
whereas disko is better for installing artifacts as a result of nixos-install, which grub needs to to work, or maybe you need multi-disk, VM testing, for the whole install flow, zfs, there's enough differences that I don't think will be reconciled so easily