Pointing out a small thing: some metal is highly reactive with oxygen, so if a disc has bad sealing, then oxygen can corrupt it.
And if M-Disc BD-R isn't especially special, and uses similar metallic setups, then it's susceptible to the same thing.
Plus all optical media written using a laser is susceptible to influences of light. That just comes with the territory. Same thing with heat and moisture, to varying degrees. Then there are scratches, issues with moisture, and the like, and protecting media is complicated.
Even with the discs properly sealed, the substrate is still porous, so air can get in and slowly work at it.
Compare to hard drives.
Hard drives are technically susceptible to losing their magnetic charge over time (unlikely) or having a physical fault preventing data access (more likely, for varying reasons, even if a drive sits unused). They're not quite susceptible to light, but they are susceptible to physical shock, as well as electric shock. Then there's the issue of moisture, and possible internal contamination over time (not likely, but possible). ï¼ The physical faults could come in with components seizing (motor fails to spin at correct speed or can't spin at all, drive can't move read-write heads, etc), or read-write heads get damaged, or other issues.
The downside to a hard drive is, I'm assuming you'll be getting something fairly capacious. Even at 250GB or 500GB, that's somewhere between 5 and 20 easily-obtained single or dual layer BD-R discs. If a drive fails, you lose that entire cluster of data. You also have to bring that hard drive online more often if you are intending to make incremental backups, which means it cannot just rest idly. This can be seen as raising the chances of failure, at least by a bit.
So I wouldn't write either one off or bring one up as as being superior, unless you take into consideration how you'll be backing things up, how often you'll be accessing it, and how you'll be storing things.
(I'm sure we've had a debate over the pros and cons of various storage media, but I'm currently too lazy to pull those conversations up. :p)
That said, which threads are you looking at which give you pause over optical media?