There is no need for a mirrored drive - hard drive failure is extremely rare, particularly for good quality ssd's. Most folks store the files locally. Not on network storage. Also, 3.5 drives are specifically available for NVR use like the WD purple. SFF is the best mix for this.
Yes you need mirrored if you don't want issues to appear/lower system uptime. I'm not using an SSD for storage - I've seen many SSDs die at work in workstations and SLC-based SSDs in servers in under 1-year of duty. Platter drive failures are more common than they should be as well - my older 2TB Samsungs are still the most reliable I have and have outlived the 3 and 5TB drives I have owned. Newer drives, many test bad out of the box, including more expensive WD Blacks and Reds. Seems we have about a 15% rate of failure on initial 1st-pass butterfly testing when we receive new-in-box drives (some SAS, some SATA).
I had one 6TB WD purple drive. It died in 2-months. Anecdotal there as it was a one-off personal purchase, not a purchase of 50+ drives like we do at work. Wasn't a great start to a newer line from WD.
I'll stick with SSD for read-heavy loads and mechanical HDD for write-heavy loads for a few more years.
Archival storage is on a separate disk-array server, backed up to a NAS and another external USB drive. I'm not too concerned with small 500GB drives for initial-clip storage in the BI system. if we can get shadow-motion under control, 500GB would store even longer duration.
I agree SFF is a great overall size for these things though. Some only have two or three SATA ports. I specifically look for at least 4. Power usage also isn't an issue - if you limit a desktop CPU to a lower speed and undervolt, you will also see power consumption close to the numbers from a NUC. Having not done any testing recently, years ago I had a Core2Duo E8400 system that I ran at 1Ghz - much faster than an Atom and peaked at around 15W usage. Atom idled at a couple Watts less, but we won't see idle/sleep state in a surveillance system. Hardware price was also about the same - the Atoms were overpriced.