It's a simple idea, but it's not quite that simple.

When it gets to be around freezing outside, you have to deal with frost buildup on the outdoor unit.

And as temperatures fall, output and efficiency generally falls. So you need an oversized unit to heat your house on the coldest days, but an oversized unit isn't great the rest of the year.

Historically, heat pumps were only good if it never got down below freezing. Now, modern cold- climate heat pumps are efficient well below freezing and Mitsubishi's models advertise that they deliver 100% of their output down to -23F/-30C. Between adding variable inverters, better defrosting, etc they've come a really long way in the past decade.

It gets below zero in the north east in the winter. Heat pumps stop working at 20-30F and the system has to switch to classic/emergency heat. They are great for fall/spring (or summer as an AC), but useless for winter.

The bigger issue is that it is extremely expensive to install ductwork, wiring for 1 or more thermostats, and a shiny new heating/cooling system in many existing homes that use classic radiator heat. Depending on where the oil tank is located, it may require removal as well (example: if it is underground, depending on state/municipal laws).

I've spent a quarter of my life in Australia and never seen heat in a house (which is nuts, because Melbourne and Hobart winters are pretty close to Vancouver winters omit the one week of snow we get here in Canada).

The benefits of having hot water and heating from the same source I'd guess

Not really. Heat pump is not creating heat unlike resistive heater. It's just transporting it. One must remember even though freezing temperatures are cold for humans, for physics and universe those are still relatively high temperature environment. After all 0 Celsius is 273 positive Kelvin degrees. So that is 273 Kelvin forth of heat to pump around. Well say in -30C, still 243 K worth of heat to pump around. So the issue isn't is there heat, the issue is the practical mechanical and thermodynamic realities of he pumping. Which in practice comes down to can you find suitable refrigerants with suitable phase change characteristics to pump around.

Which for normal ambient temperatures on Earth is "yes". Just usually matter of how high pressures one has to use and other nasty features of the materials. For example recently even just CO2 has been started to use more again, issue with it mostly being it has to operate at higher pressures than more traditionally used refrigerants. They just don't call it CO2 in the bizz, it's refrigerant fluid R744. That works down to -56.6 Celcius given its triple point temperature. So it won't heat one in antarctic -80C winter, but for most of Earth even in cold climates -56.6 C is plenty. Problem just is it has to always work under high pressure, since in ambient pressures CO2 just sublimated from solid to gas. Pumping around solid blocks of dry ice isn't very convient for continuous heat exchange process machine.

Which adds some cost to the pump components. On the other hand... CO2 is pretty darn harmless. As long as concentration locally isn't too high, humans, animals and plants are perfectly used to handling the gas they exhale. It is non flammable, it is green house gas, but it is green house gas we naturally exhale. Some unit leaking it doesn't change much, since usually CO2 for industrial use is extracted from waste product gas, that would end up in air anyway.

Plus on need be it can be distilled from air, it's just energy intensive. Which is why "carbon capture" isn't a bigger thing. We know how to do carbon capture. It's just energy intensive and thus on climate impacting massive scale energy prohibitive.

A split air conditioner is will generally have an indoor component (evaporator) and an outdoor component (condenser).

When the compressor runs, the evaporator will get cold and the condenser will get hot.

A heat pump also has a indoor and outdoor components, along with a new item called a reversing valve. The reversing valve allows the system to switch which component is the evaporator and which is the condenser.

When the compressor runs and the reversing valve is enabled, the evaporator is inside and you get cold air.

When the compressor runs and the reversing valve is disabled, the condenser is inside and you get hot air.

Mini split you mean? Those are also heat pumps but used in homes that don't have duct systems installed. So theoretically you get large mini splits on each floor to heat or cool all the spaces. Downside being if you close a door and there's no mini split in that room, it will be more affected by the outdoor air.

I may be way off here. But I believe a mini split and heat pump are the same thing? At least where I live the term is interchangeable.

"the" video? He has many on heat pumps. Get watching!

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Just had a 48btu unit installed. 2,000 sqft, 3 - 9k wall units, 1 - 18k ducted unit. Operational down to -22 F.

$20,000 and then a $4,300 rebate and later a $2,000 tax credit. Down right affordable. Would have been even less if new ductwork didn't have to be retrofitted.

There are many rebates for their installation. Check your city, state, federal, and utility for kick backs. That's having a huge effect on demand.