Backups get worse over time, keep these in mind when designing your backup strategy.
Good backups cover your working set. If you accidentally clobber something you're currently working on, you can get it back with minimum fuss. A good backup also provides you with some protection against catastrophic drive loss or other wholesale catastrophe. Good backups generally have an ongoing monetary cost, so there's a risk that your backups will fail not because of hardware failure but because you missed a payment.
Bad backups can look like good backups until they fail to restore. Your bad backup would be a laptop backed up to a hard drive in the same house, both at risk of the same catastrophe like a fire. Bad backups are better than no backups if you're lucky, but you're not really doing this to be lucky.
Ugly backups are the worst. Imagine a decade old laptop which has had several attempts at file system organization. There are financial records mixed in with old photos mixed in with ancient versions of free software. You're not sure whether to throw the whole thing into the chipper, or to back it up very carefully. It will take too much time to sort through everything, much longer than it's worth. And yes while the drive is backed up carefully it's on old hardware that's expected to fail sooner rather than later.
Good backups consume time and money, but given enough money, there's not that much time needed to keep enough copies of your current working set of files active and alive. Once the working set grows - every photo you ever took, everything you ever downloaded, all those records of your existence - it gets harder. I don't have an easy answer for the "ugly" scenario except to do all you can to prevent it from happening, and a strong recommendation to sort through things up front rather than let random files clog up your drives.
For myself, the most important backup question: did I save enough pictures of my younger son as he was growing up? His older brother got all of the first photos on the first digital camera (backed up to floppy drives). Not so sure of the second child's digital record.
Also a sad reminder for those of us from the Napster era: we had very interesting digital music collections, and a lot of those digital files were "too big" at one point in the history of storage. Shiny new streaming services combined with ugly on-disk storage techniques means we just don't have that music as bits on disk anymore.
