The other one is that a lot of people presume a house needs state of the art insulation, which obviously costs a lot to retrofit.
However in nearly all cases the old radiators work just fine with a heat pump as does the insulation. Underfloor heating and more insulation would make it more efficient, but that is often not worth the money, unless you do plan to do it anyway. When you redo the floors you can then add underfloor heating. Same for insulation, once you redo the roof or have to do work on the walls.