When you think about disks -- thousands of complications come to mind: starting from the disk itself (+/-, DL, RAM etc...) to doubt about the data written on the disk (data corruption) caused by 1000s of possible combinations of problems (dirty lens, bad mechanics, old/faulty driver, manufacturing defect, software fault, writing speed, wrong way of handling, a fingerprint or scratch on the surface etc...)
Following all these facts, it's a matter of chance why a disk should write properly, and in most of my personal experience, they never do, even if they do, they degrade quickly over time and are susceptible of things like fungus etc..
Then it doesn't have any stable shelf life either, at most, it'll be 5 years; i.e you've to rewrite all your disks every 2 years in order to ensure that your data is OK.
Well, after all this, considering the amount of blank disks you waste and the data loss that occurs, disks ARE the most expensive and most unreliable medium.
A much better alternative is to buy hard disks for archival purposes or use tapes.