For the most part any type of device that uses resistance heating has to have the heat that it generates conducted through to the needed area.
In electro-cautery the heat needed is GENERATED by the area. In other words the tissues become the element that heats through resistance heating.
Thus, the temperature is even and is controlled by the voltage and the current generated. Electrocautery units have a continuous temperature. Furthermore, the heating is limited to the area of the arc. Surrounding areas are not damaged. Tissue damage must be kept to a minimum. All you want to do is seal off bleeders, not generate areas with diameters in excess of 3mm with 3rd degree burns. Smaller areas are less prone to infection, especially since their blood supply is so rich. (they are blood vessels)
Devices like soldering guns are bad because they lack the thermal loading capacity to maintain their temperature during operation and they make too much tissue damage.