What fraction of the population lives with -40 degree winters? That's the only reason they might not be viable, in that they require customization that is only useful to a tiny tiny tiny fraction of the population.
In terms of cost, it's a lot cheaper than gas + electricity (it evens-out on the coolest days, and is cheaper otherwise). It will not amortize the cost of the heat pump itself for a while, but that's a cost I'm happy to eat right now.