Drives constantly failing, different brands, all on HCL

And how about a problem with the SATA cable of the PSU. If the disk were to receive less tension that it needs when writing, write could be partial and interpreted as a bad sector, couldn't it ? that said that's really stretching things, never seen a PSU doing that, they were either working or not working at all.

I don't see for the controller either, AFAIK it doesn't control head parking or things like that, it just acts kind of what ethernet does for a computer, that's to say a transport medium with a communication norm on top of it. So yes you could have problems with corrupted data but not with bad sectors. Well unless the tool to detect bad sectors got corrupted data from the disk. I don't know how SMART works but I think the disks does it by itself and then sends some information to the NAS for it to show them to the user, could a corrupted feedback show errors that don't exist ?

testing in another system is the best option indeed, you will know for sure if the disk works with the manufacturer utility.