In the last few years, I have seen many people confuse replication with backups. People see them as the same thing, but they really aren't. Even with snapshots, if the devices are the same, they might have the same firmware bug, etc.
Just to further explore that a bit, would you say replication adds independent copies for failures of media, while backup adds copies made by independent software against failures of process / software / media.
Replication increases risk of data loss when implemented incorrectly, because added resources increase the probability of bit errors. This applies to both replicated disks (RAID) and servers. Replicated servers must use ECC memory as well as checksum blocks and periodically scrub data to ensure integrity (e.g. what ZFS does for you). If they don't then a bit error corrupts the data on all servers, because you have no way of know which copies are pristine or how to piece together pristine parts.
