As long as you understand that the economy models are basically a separated window air conditioner. The hotter outside it is, the more heat they move, both in heating and cooling modes. The units that are the cheapest typically have a balance point a few degrees above freezing. Some of them will shut down below about 40 degF or 5 degC, some of them will switch to backup heat. The worst ones keep on trying to heat and defrost cycle often. So just when you need them the most, they fall on their faces and cost more to provide less heat.
FWIW, all air source units suffer this symptom. The inverter units with smart defrost do the best they can, given the environment they have to cope with. When the environment isn't so harsh, the single speed, cap tube metering units only have maximum performance at a single set of conditions. The variable speed units with expansion valves work hard to save money and move lots of heat.
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