I do have only a handful of BD-R DL 2x MEI's under Sony branding burnt 6 years ago - they seem to be okay but they do have some sleeve patterning on them now. Readability seems okay but not perfect, but I can't really bother to scan.
I find the sensitivity of the data layer to the patterning on the sleeves to be quite irritating. There's a chance that stacking them on top of other discs with printable coatings could also cause issues because there's some roughness to them and I've had some cases where some flecks of printable coating start to stick to the disc above necessitating cleaning.
Of course, spindles are a nice way to keep them if you don't go through them often and won't have the patterning issue with the sleeves. However, because I did get some discs "randomly" sent to me in sleeves from eBay sellers in Japan, I didn't keep enough spindles for all of them. Too late for most of my collection of over 350 discs though ...
That being said, optical media has lost my favour because of speed and cost. Assuming your time is worth nothing, and the drive is worth nothing either, most BD-R SL discs don't sell cheaper than about AU$0.80/disc in bulk.
On a $/GB basis, assuming you fill every disc fully, this works out to be 3.2c/Gb.
A WD Green 3Tb drive goes for 4.3c/Gb, which is about 34% more expensive, but hardly breaking the bank. If your discs are averagely filled to 18.6GB, then your effective media cost is the same. BD-R DL is even more expensive.
However, when you factor in the time, burning 3Tb of BD-R SL discs (120 discs) at 8x (12 minutes a disc) will cost you exactly 24 hours. Doing a 3Tb drive fill only takes about 7 hours. The cost difference in media alone is $33 for 17 hours of extra hassle (under $2/hour).
That's before you factor in the discs volumetrically take up more space, and creating a second copy will cost you a similar amount of time.
Of course, the idealism that discs can be read by any drive, and has "read only" safety is practically undeniable, but I've had enough issues with data loss on BD-R - some permanent thanks to Ritek, that I cannot justify further investment in it. At least a hard disk is rewritable, and you can produce parity files to cover for "bad" sectors if the drive hasn't completely failed.
My experience with hard drives is generally more favourable. I've carried home ex-business Maxtor hard drives of 120Mb from 1991 that still work just fine today. Not to say that they all do, but I haven't suffered major data loss with hard drives in general.
You can flag partitions to mount read-only (subject to the OS' compliance) - e.g. for NTFS this involves some diskpart commands.