It has to do with resilvering (array rebuilding) and encountering UREs on the resilver on these large TB drives which destroys the entire array. RAID was never designed to work with the capacities of mechanical drives we have nowadays. If you need RAID on mechanical drives then typically you want RAID 1, 6 (slow as hell), or 10 unless you have some real strange requirements.
Do a Google search on URE and RAID 5. Tons of stuff. RAID 5 is depreciated for anything other than flash storage.
I wouldn't think that NVR has to run in RAID 5 if you give it 4 drives. I'd be shocked if it was required. But I'm also shocked that they list 4 x 8TB in RAID 5 as a "selling point".