17°F isn't the point at which my heat pump stops working. It's just the lower temperature conventionally used as a point of comparison on the spec sheets for such devices. There's another column on the spec sheet for 47°F (where my heat pump model is listed as having a CoP of 3.6).

And as for the question about colder climates: it's not a matter of whether an area can occasionally experience temperatures below eg. 17°F. What matters is whether you'll ever experience daily highs that are cold enough to prevent heating the house.