The reason heat pumps have traditionally been untenable is that they are prone to icing over when below freezing. If the coils are covered in ice, then air can't be blown past them to get new heat to pull in.

Further, in many cases, the heat pump wouldn't keep your house warm during the coldest period of the year, which meant you still needed a heater. Not a huge deal since you probably have an AC for the summer anyway.

The issue as I understand it, was that the thermostats were not smart enough to know when to quit and so a heat pump would run until it literally could not keep up with demand, when it should have instead switched over to gas heat some time earlier than that when efficiency dropped.

Modern heat pumps deal with all these factors to the point that I expect them to become standard fare as current ACs get upgraded over the next decade or two. I know that if we ever need to upgrade our AC it will be replaced with a heat pump.