Those points don't change the fact that you (and everyone else) should test back ups periodically.
If it's not file system permissions nor other configuration problems then it will be a failing storage medium or just dumb user error. So the only way to be sure you have a working back up is to test that back up - ideally before you actually need to use it.
I got a little lost while writing. But indeed my point was double check! Because in my case everything looked fine at first sight.
This is even why I don't use the same process for my two backup (daily/monthly). Because to be fair you can't constantly check that your daily/hourly backup isn't corrupted, you rely on its build-in safeguard and occasionally you check it.
But by using two different methods for daily vs monthly(offsite) backup, you significantly reduce the odds that the two different methods failed at the same time while you need them. (Also a monthly full clone is way easy to check than a Time Machine Disk)
Yeah, once upon a time at a startup, I wanted a file restored. We were small; the CEO was actually the one doing the backups (to QIC tape cartridges).
Of course, the recovery failed. In fact, the backups hadn't been working properly for months. I lost a few hours of work; if our file server had actually failed, not having a good backup could have literally been catastrophic. As in would the company have survived?!
I recall an ancient quip, more or less: "if you don't test your backups, you don't have backups, you have dreams".
If it's not file system permissions nor other configuration problems then it will be a failing storage medium or just dumb user error. So the only way to be sure you have a working back up is to test that back up - ideally before you actually need to use it.