A RAID can be lost or corrupted, or someone can overwrite or delete a file.
And tapes can be lost or corrupted, or someone can burn the building down.
This is an old argument, and every time it gets revisited RAID starts to look better. Overwriting / deletion might have been a concern prior to modern filesystems which incorporate easy and cheap snapshotting, but nowdays that part of the argument just doesn't fly. Corruption is still a concern but, again, that's a risk you take with any backup solution too.
There's no such thing as a guaranteed backup. If you're very rich and very paranoid, you could certainly rig up a "backup solution" that involves copying your data every 5 minutes to 50 different offsite locations in 50 different countries, plus having some cheap third-world-labour transcribe all the zeros and ones to a paper copy for storage in an underground vault. And even that's not 100% because a really big asteroid will result in unrecoverable corruption. In the end it all comes down to how much you're willing to spend and what level of risk you're willing to accept. For most of us who aren't running IT departments that equation comes down to something like "ZFS RAIDZ2".