Works great until your computer/home/office catches fire.

Personally I have a mirrored drive set that I use as my primary storage, and a third disk that I rsync to nightly and use for backups of other stuff, because I don't trust that I won't have filesystem errors or controller problems that might render both disks in the mirror unusable. Then on top of that I use Crashplan to ensure I have an offsite copy.

For most of my code I also push to github or private external repositories regularly and/or pull to my laptop.

Maybe a bit paranoid, but I've had to recover from enough scary hardware and software failures over the years to not want to take risks.

Or use TimeMachine if you are on a Mac.

No, if TimeMachine screws up a file it stays screwed up for ever after every backup, there's no integrity checking. It also completly barfs on large files that change little by little. The best example of that is the entourage mail store, but any single file db will do. I've seen clients bit bad by these problems. Don't get me started on the time capsule...

Option-click on the Time Machine menu to verify backups. Sleep well.

What should we use instead?

Linus's solution is to write software that people run and use daily.

Point your Time Machine backups at a NAS that's using ZFS with block-level deduplication.