Sure if all machines go at once, you're screwed. But I'm not too worried about that, personally. If you have something you absolutely can't lose just stick it in a Backblaze bucket and use syncthing for the rest.
Backblaze is cheap and has both an API and web interface.
Yep I think Backblaze for backup paired with SyncThing for syncing is probably about as good as it gets. Admittedly my needs for syncing aren't crazy but it's worked well for me.
And Syncthing repeatedly and very explicitly warns that it is not a backup system, and you should have a separate backup running as well.
"Run it on your desktop computers and synchronize them with your server for backup."
For those who don't mind a little extra work (I argue Backblaze is almost no work using their Web UI), syncthing is a really great solution.