The old ones were efficient down to a certain temperature, then efficiency fell off quickly. They turned off or used resistant coils inside below somewhere around 32F?

The new ASHP's work on various inverter designs and with other new tech are amazingly capable.

Basically as the temps drop, the pumps run more constantly and even full out and deliver less heat(as mine is doing this am at -20F).

I installed an optional pan heater in my outdoor unit. The reason is in our climate, the outdoor units can frost up as the coils get super cooled and snow or ice collect. The unit senses this and automatically defrosts. An expensive move! But it doesn't happen often nor is it a part of regular heating cycle.

ASHP are still not a heat panacea. They belong in very efficient spaces and in most cases, still require a back up system. I have 2 stand alone pumps in a restored house. No back up, so far, so good!

You can see this graph claims this model of ASHP is 100% efficient(equals 300% ouput!) down to 5F. Even at -10F it's 75% efficient, way more efficient than oil.