I will never use an internal drive as a backup. A few years ago i had a surge strike happen both my internal drives went but my external drives had no damage.
I find it very convenient to use an internal drive for Time Machine backup; however, it is not my only backup.
Once a week I make a full bootable backup on an external drive using SuperDuper. I keep that disk in a Pelican case in a separate building from my computer.
Time Machine keeps copies of whatever changes during the week, and beyond.
Time Machine isn't a great backup solution because it requires a lengthy recovery process in the event of a total disk failure. The Time Machine disk is not bootable. It is a great way to recover a file that was accidentally changed or deleted and it requires no user action to stay up to date.
SuperDuper creates a very useful bootable backup that will quickly have you up and running after a disk failure.
The two are complimentary and I recommend using both for comprehensive and convenient backup.