Chris

Yes, they have to work harder the colder the outdoor air is - there’s less energy in the colder air so the pump has to use more energy and drive more air through itself in order to achieve the target temperature indoors. If he house is poorly insulated and prone to losing heat easily then the problem is all the more acute. The UK’s particular problem is that we have been spoiled by cheap energy (abundant coal, then natural gas) so we have had no particular incentive to spend on energy efficient homes. So before you install a heat pump you have to make sure the house is well insulated.

Service and spares are hard to quantify in any meaningful way because those costs come down with economies of scale just as the installation cost does. But the machinery is not fundamentally difficult. How often does your fridge/freezer break down? It’s the same principle on a smaller scale.