i'm no expert on timeshift; one of the experts is Chris probably a few others on here ! To me there is a bit of a cross over between getting an OS back up and running when some of its system files get borked or an update somehow causes a problem , cloning of the OS and lastly mass storeage of files which are not system files.
I've done back ups in several ways depending on my mood , memory and last thing i was researching.
Probably one of the most important is a means of getting your OS back up and running if some update causes problems. For that I use Timeshift from the command line ,with a one off use of GUI wizard for where backups are going to be installed.
if i enter into a snapshot i used timeshift to create i can give you an idea of whats saved:
So i think put simply Timeshift might be considered similar to Microsofts "restore" or whatever it was called.
Now for specific backing up of say directories such as specific directories in Apache web server running local host located at /srv/http (im on Arch) I like to use the gui of rsync . I can do i quick dry run then back a whole web site in dev to an external hard drive. When i was on slackware and wanted to backup /etc/ files i just used "ls" and piped through tar i think it was. Thats was so i could refer back to from working files if i messed up on editing something like /etc/httpd/conf/ httpd.conf
I've also played with fsarchiver to save whole partitions and you can use ddrescue to save a whole partition as an .img file
you can also use dd command , i havent done it but :
so this should be possible to a usb (big enough) :
# dd if=/dev/sda of=/mnt/point/sdaimage.img