It's not necessarily about 100% being the best possible heating efficiency^1^, but rather that the exact method is irrelevant. Any two pure electric heating methods will always, by definition, be equally efficient at heating a space.

Also, not dispering the heat isn't really an issue in continuous long term operation. Free convection and radiation will eventually disperse the heat for you (heat in = heat out in the long term equilibrium is a fundamental law of physics, and for a given (mostly) matter tight space in given exterior conditions, the heat out is determined exclusively by the average interior temperature. Same amount of heat in must always lead to the same average internal temperature in the long term equilibrium), in fact the vast majority of european homes, which often lack central ducting, rely entirely on passive heat distribution in their most common heating methods, under floor and radiators.

1: And using the term efficiency (useful work out/total work in) isn't really accurate for heat pumps, the correct term is coefficient of performance (cop = total heat moved/total work in). Calling a heat pump 400% efficient is a bit like calling a fuel truck 1000% efficient, because it used the energy in 10 liters of diesel to move the energy contained in 1000 liters of diesel down the road) The useful work a heat pump is doing is the act of moving heat, not the amount of heat moved, and since the act of moving heat theoretically takes zero energy in the ideal case, see maxwells demon, technically heat pumps all have zero efficiency