While old (2011) and mostly based on enterprise drive stats, this tom’s Hardware Investigation: Is Your SSD More Reliable Than A Hard Drive? article would seem to indicate that there is more variation between manufacturers than between the technologies. It also highlights that manufacturers’ reliability data is misleading at best.
I had difficulty quickly finding more recent mass reliability data that was independent of manufacturers’ figures. Could you please provide some stats for your statements?
Re backups, HDDs are definitely the cheapest way to go, but if I could afford it, I would use SSDs in my NAS boxes, just to be able to transfer data at GbE wire speeds all the time.
Re RAID, HDDs are cheap enough to only use RAID1, as recovery from a failed RAID set is much easier from only one drive than having to temporarily mount a multi-disk set in a computer. However, the hidden side of RAID (or any redundant topology) is the re-build time, during which the set is down in peak performance.
Windows Storage Spaces appears to have comparable performance – though much better write speeds – in its redundant modes, and the drives can be read on any Win 8.x computer as is, rather than use cumbersome RAID recovery software.
When we get to having enough income streaming in again, I will re-consider our NAS setup, looking at dual NAS boxes in active-passive HA mode, that effectively act like RAID1 together, but with unit redundancy, rather than just drive redundancy. I would still keep my current two units as backups for the backups, each 24 hours behind each other.