I take your point, maybe if it wasn't for the potential for a leak, we would have an oil filled central heating system! However, without research, I still think that a carefully controlled system of electric radiators meeting a precise need might well be more efficient.
It may just be in the UK, but I have a feeling that, despite making the engineer's life a misery, my boiler is doing very little less water heating in the summer, than the winter, in my mind I think that these systems are thinking that we are all taken luxuriant baths, twice a day - this era ended when my daughter moved out, it's now her beau's problem!