There are 4 different model variations of the HC520. The one inside the WD Black D10 is the worst of the 4.
That doesn't seem that great? You could have had brand new 12TB HC520 with a 3 year warranty for £180 inc. shucked from WD Black D10 drives in the past couple of weeks.
NAS drives have a feature to work more reliably when used in a hardware RAID set in a server (e.g. in a data center). By hardware RAID I mean the server motherboard has a dedicated RAID chip. 90% of consumer NAS do not have this, they have software emulated RAID so there is no benefit.
If that’s the case then what’s so special about a ‘nas’ drive compared to any other drive?
If it has a case fan like a PC then yeh. 7200rpm can get toasty.