Just curious about Ubuntu One- for example, if one day, my computer wouldn't be able to boot, would everything be backed up safely? Or if my hard drive became fried or whatever- would it still be safe, or would it realize the files were gone and delete them from the cloud?
Yes, your files would be safe in both of those circumstances.
But as you rightly allude to, Ubuntu One isn't a backup tool. If you accidentally delete a file, that file will also be deleted on the cloud.