It is more reliable. Although having an internal watchdog timer to a microprocessor is a low-cost option, the problem is, it is often subject to the same transient errors as the microprocessor as it operates from the same supply voltage and clock inputs. And these designs are also vulnerable to software errors such as runaway code, which can disable the built-in timer. An external watchdog timer is more RELIABLE as it guarantees proper inputs to the processor each and every time.
