Yes, absolutely. The people who’ve had them fitted have seen that they work a lot of the time, but as you say as soon as the weather/temps overwhelm the capacity of the install (as they haven’t done the associated insulation improvements or changed the radiators to ones more suited to the lower water temps) then they call him to complain the house isn’t warm enough, and it’s costing a fortune in electricity.
I’d love one, but the 150 year old house I live in would require a lot of expensive work for it to take the place of the oil boiler we have. It is getting to the end of its life though, so need to make some decisions in the next couple of years.