Earlier quoted context omitted.
If you are making sandboxes, you need to put the files in place each time. With ZFS clones, you can keep referencing the same files repeatedly, so the amount of changes to memory needed to create an environment are minimized. Let’s say the sandbox is 1GB and each clone operation does less than 1MB of memory writes. Then you have a >1000x reduction in writing needed to make the environment. Furthermore, ZFS ARC should…
It's worth noting that if you go down a layer, LVM snapshots are filesystem-independent.
That said, if you really want to use block devices, you could use zvols to get something similar to LVM2 out of ZFS, but it is not as good as using snapshots on ZFS' filesystems. The write amplification would be lower by default (8KB versus 4MB). The page cache would still duplicate data, but the buffer cache duplication should be bypassed if I recall correctly.