I have a Nurture Right 360 and we were 7 days into an incubation of 22 eggs when the Blizzard of '20 struck and caused massive rolling blackouts throughout Texas for 5 days. Our electricity would be off for 10 hours, on for 1 hour, off for 15 hours, on for 45 minutes, etc. When power was restored, my first impulse after straightening up the driveway, the woodstove, the leaking irrigation pipes and all that stuff was to chuck all the eggs because, surely, they're dead.
My wife convinced me to let them continue in the incubator for another week and check them for signs of life. We ended up with something like 16 healthy chicks from that hatch. I was amazed.
I think they cope with off and on low temps better than they do temperature spikes.