At the risk of asking a dumb question, doesn't working great 99% of the time represent a pretty good solution to most problems by any standard? The tone of the post you're replying to seems to suggest having a backup heat source for rare situations is somehow an indictment of heat pumps, as if addressing your climate control needs 360 days a year without relying on fossil fuels is somehow a failure. Sure, those few days where an alternative is necessary means you might need a backup plan. But that's light years ahead of having to rely on the backup plan every day.

I'd also suggest that the necessity of a backup plan can be reduced as well. Having an unreliable power grid is probably not an immutable law of nature so much as a policy choice and modern heat pump technology performs well at considerably lower temperatures than you describe.