STM32 external voltage monitor / watchdog - worth it?
Well, an external watchdog is immune to you messing up your code and has more flexibility. An external watchdog is also more suited for a system with multiple processors/FPGAs that might need to be under the purview of a single watchdog.

So...it depends...Usually for simple systems or system where safety or catastrophic failure is not an issue, an external watchdog is not worth it. For example, if your MP3 player freezes nothing bad really happens. But if the inertial measurement on your UAV stalls...that's kind of a really big problem.