This link is misleading which causes misunderstanding
The ATmega328p have watchdog timer build in, which if enabled uses an internal RC oscillation, and this will for sure reset the MCU, if the timer isn't reset by the user program before time-out. it works rock stable...
Something that can cause unreliability is that brown-out detecting of power voltage, will also cause a shutdown, and if the designer isn't aware of stable power voltage, then you get unstable MCU system...
Below snippet is from above ATmega328 spec:
Please note there are 3 Operating modes:
– System Reset
– Interrupt and System Reset
In Interrupt mode, the WDT gives an interrupt when the timer expires. This interrupt can be used to wake the
device from sleep-modes, and also as a general system timer. One example is to limit the maximum time
allowed for certain operations, giving an interrupt when the operation has run longer than expected.
In System Reset mode, the WDT gives a reset when the timer expires. This is typically used to prevent system hang-up in case of runaway code. The third mode, Interrupt and System Reset mode, combines the other two modes by first giving an interrupt and then switch to System Reset mode. This mode will for instance allow a safe shutdown by saving critical parameters before a system reset.
[EDIT] below removed as it was not correct, comment from mfalkvidd is correct, that mysensors is using WDT for wakup-call