If your use case is running a single container, never creating new ones and never making a snapshot and that you don’t need to be able to apply quotas, then dir is probably the best option.
For just about every other cases, you’d want ZFS or btrfs as that will drastically reduce the disk usage when running more than one container, cut down the container creation time from dozens of seconds/minutes to miliseconds and allow you to do things like snapshots, fast backups, migration, … which all would rely on slow rsync otherwise.
ZFS on pure filesystem performance even on a dedicated driver will be slower than directory (ext4/xfs), so again, if your use case is a single container, no quotas, no snapshots, no migration, no backups, … then ZFS isn’t for you.
But the most common use case for us are users creating more than one container, often using them as throwaway environments and for that, having to go through a full unpack and complete duplication of disk usage on ever instance gets quite frustrating.