Chorus came around and installed fibre at our place a few weeks ago, the last thing the technicians said after the installation was that we should turn off the power outlet for the ONT if a power cut lasts more than 2 hours, or the ONT might blow up when the power comes back.

Anyone can give some sort of scientific explanation to this?

We used to be in a suburb where WorldExchange/Xnet supplies fibre connections (not UFB, not sure what they call it), the fibre modem and all that were built-in with the new house and we never had such warning from xnet guys, only once xnet called up and asked us to restart the modem for update, is that because the technology is different between what xnet does and UFB?