Simple, yet reliable "family backup" strategy

So far, just about everyone to whom I explained the following backup strategy thought, that it was a really good idea. I was just wondering if there are any downsides or perhaps even flaws aside from the things that I'll mention.

The task is the following: Among my family and friends there're a lot of people using computers (always some Windows), some people have more than one computer. They're looking for a backup solution that's simple yet offers perfect protection from ransomware. There's no need for encryption, they're even fine with the need of a manual recovery (which will be done by me, anyway). They can follow simple instructions and decide for themselves when to create a new backup.

The most important idea behind the following strategy is: Never ever let the (possibly infected) operating system write on the backup drive. Not "just a short time during backup", never! My first idea was having a NAS "pull" the backup from the computer (instead of the computer"pushing" it, but then very few people have a NAS. However, all of them have USB drives...

So I take all of those drives and install a small linux on the first partition (the second being for the backups) and make them bootable (via USB). Upon every boot they run a script that first identifies the machine it's currently running on by using some hardware characteristics or whatever, mounts the partitions to be backed up (mostly c:users, maybe some others) and creates snapshots using rsnapshot on /backup//snapshot.x of those files. If that's successful, the computer will be automatically powered down. This can be done with multiple USB drives and the people can decide for themselves how many snapshots they want.

The obvious disadvantage is that all of this has to be configured manually. The fact that the PC needs to be able to boot from USB usually isn't a problem. Recovery is manual work, too, but I assume that it's rarely needed and since it's family and friends I'd have no problem investing that time.

On the other hand it seems to me a rather easy "fire and forget" solution: Power down your PC, plug in the drive, power it on, watch it boot, go to bed. It should be absolutely reliable as long as you have at least one USB drive. (The users understand the implications of having one or more backups.) It can easily be used on more that one computer/laptop and - theoretically - works with any operating system that some Linux can mount. It might have issues with encrypted or compressed files (not sure), but they're not used anyways. There's no privacy if the drive is being stolen.

Anything I missed? Any other thoughts or suggestions?

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