I'm not sure "crash" is the correct term. Upon power restoration, the NAS will come up cleanly and only alert that it had gone into safe mode due to power interruption. I don't believe any fault will be reported by the pool(s)/volume(s).Otherwise it’s a crash if the power outage is longer than the UPS’ scheduled shutoff time as configured on DSM.
Here's a crappy old video, but the end testimony is what matters...