All internal watchdogs are sleeping during power up
and can only be enabled, if power up reset was done
right. Also wrong software can disable it again.
So I found they generally useless.

I use only external watchdog, since ever watchful.
Most of them include also ever right working
power on reset to start the micro in every case of
power up (simple RC-circuit works not so).

E.g. look at TI: TPS3824 or MAXIM


Peter