The rating I was referring to was the "guaranteed operating range" of -25c to 21c on that page.

We dropped below that on Friday. The tech that came out said he was getting a bunch of calls with the same problem.

It wasn't all of them, but does seem to confirm that the -25C threshold is when the probability of failure increases.

We have a Mitsubishi hyper heat as our only real heat source. It was working hard Friday evening but our house was still warm. Sometime Friday night it stopped running. The unit is rated down to -25C/-13F and it got colder than that. It ended coming back on Sunday when temps got up to 0C/32F.

We've had the unit for 6 years and are still happy with it. Since our house is very well insulated (HERS score in the 20s), even with no heat for a day and a half, we ran a small ventless propane heater in the basement and a couple space heaters in the rest of the house which kept everything around 13C/55F.

If the house were not as well insulated, I'd be more hesitant to have the heat pump as our only heat source.

We're not sure what caused the shutdown (some internal protection switch or something froze). A tech was able to make it out on Saturday but couldn't find any issue. I'm fairly certain we've had a cold night like that before, but IIRC we ended up turning off the heat overnight so it didn't need to struggle and let the house drop 10 degrees or so. In the morning the heat started right up when we turned it on.