I haven't installed one but am about to take the plunge due to the tax credits available this year. So I've done a deep dive into the pros and cons. When you say "hybrid," are you referring to a single outdoor heat pump that heats the home AND the water? It's worth distinguishing, because yeah -- in general, air source heat pumps become marginally less efficient as the outside temperature drops. Just as in deep cold it might take said heat pump longer to raise your indoor temp by X degrees, it also takes longer to raise the temp of a tank full of water. When I see complaints online about heat pump water heaters, it causes me to wonder if people undersized the tank -- like, did they get a 50 gallon when their use patterns would've been better served by a 100 gallon?

I have an air source heat pump already that heats and cools my house. We also have an older gas boiler that does radiator heat as a backup. This winter, we used the boiler/radiators on exactly 20 days total. The heat pump covered the rest of the winter, no problem. But interestingly, I found that I saved money on the heat pump by eliminating programmed nighttime setbacks on my thermostat -- the heat pump worked more efficiently by maintaining temps over time than by having to raise them quickly in the morning, at the coldest part of the day. That kind of "high inertia" performance (for lack of a better term, I'm not a scientist) would indicate that maybe sizing up to a larger tank would be a good idea for a water heater.

I'm now looking at getting a *separate* heat pump water heater to put in my basement. The cool thing about these is that they are effectively air-conditioning your basement in order to heat your hot water -- they just move heat from the surrounding environment into the water in the tank. So in this region, I think there are some extra benefits in that a basement HPWH can dehumidify your basement and effectively suck up waste heat from boilers, hot water pipes, dehumidifiers, dryers, etc. But it does make the basement itself colder in those winter months.