Are DVDs are more unreliable than CDs

Because DVD tracks are more closely spaced than CD tracks, there is much more likelihood that they will not read successfully in another reader or even the same reader. There is a difference in the technology involved but you are essentially storing far more data on a DVD than you are on a CD of the same area and so the accuracy of the machine that reads and writes is more important.

Rewritable discs are always less reliable than Write (only) and it does pay to buy the best quality branded discs for anything you want to store long term.

If you take reliability and long-term storage into consideration, CDs are probably still cheaper (byte for byte) but it's notable that most people (commercially) who set great store by their backups still use DAT tape.

The alternative is to invest in a second hard drive installed as a slave and to only use it for backups (an internal hard drive is cheaper, quicker and usually more reliable than an external hard drive).

The other thing worth noting about backups is that only data is worth backing up. Anything that can be reinstalled (such as your operating system and applications) is only worth backing up so that you can restore any settings. If you have all the install discs (and the updates), it's a waste of time and disc space to back up the whole of the Windows and Program Files folders (assuming you are using Windows) because, if you have a crash, very little of it will be reliable or meaningful when you reinstall (as you very likely would have to do). Backups are really only useful in the case of data loss. It is quicker, cheaper and more reliable to format and reinstall the OS and application SW and then to restore the data than to resort to a system backup which may have been taken at a point where the system was already corrupted.

I personally use CDs for backup/archiving and try to ensure that all my software is available and ready (i.e. up to date) if ever I need to reinstall.