The ink on writable optical disks fades over time... So of all the copies were written at the same time, they might all stop working at around the same time and cause problems :)

Yeah if you're really taking this seriously it's worth investing in an LTO-4 drive and some tapes.

There might be a better medium available nowadays but if I seriously wanted to have a piece of data fifty years from now that's where I'd start.

Really, any second form of backup would be a good idea. Preferably, it should be stored offsite and separate from any other backups.