There is no completely reliable medium. Each one has its pitfals. Hard
disks rely on a complex mechanical system that will probably be
completely replaced by solid state drives in a few years. CD's depend on
some dyes which are subject to chemical degradation. Flash memory rely
on electric charges remaining at their seats...
Oh, so sad that nothing is perfect :) You'd think that after all these years ...
BTW, I replaced the boot partition on my computer with a small and cheap SSD. Wow, what a difference! Highly recommended.
A good quality CD-R may survive 15+ years. I mean those with azo dyes.
Yes, it MAY. Then again ... it may not.
Cloud backup is becoming a possibility as well. You do have to have a good pipe to upload, and have faith in your cloud provider. Will they be around in 10 years (doubtful) and will they actually keep your stuff safe (probably, for short term). On the other hand, your own backups might not be all that safe either: what happens if there is a flood or fire?
I used to be a big fan of CDs. But that was then. Have a look at new computers and see how many don't even have a CD drive. I read somewhere that new cars are starting to come without CD players. My latest car has one, but never used it. Just use a USB stick with lots and lots of music on it.
But, I think the key to take away from this discussion is not to rely on any single backup of anything. Things will go wrong when you least expect it!
The good part of all this is that we're a lot better off than we were 10 or 20 years ago. Media is much cheaper and more reliable. No more excuses about not having backups!