Yes, what's more difficult is implementing data checksumming without CoW. What you may not be used to in ZFS is a nodatacow option (per file or per filesystem)

Yes, what's more difficult is implementing data checksumming without CoW. What you may not be used to in ZFS is a nodatacow option (per file or per filesystem) which does exist on Btrfs and it implies nodatasum (no checksums for data, metadata is still always checksummed and cow). Conversely, nodatasum does not imply nodatacow.