When the point type ignitions used a resistor in line it reduced the current flow through the coil and ignition points, next step was dual points in an effort to increase coil saturation and prevent the points from burning. At the same time it was common to mount the coil horizontally on the intake manifold. There was seldom a problem with the coils at the time because of the reduced current flow on the primary side of the ignition.
With the advent of MSD, Crane, Acell and others being able to switch higher current with electronics insted of mechanical points the coils were now receiving a full 12 volts instead of the previous 8-9. Often the older type coils will overheat and fail. New designs were created and are more efficient and produce higher secondary voltages. If the older type (
oil filled) is laid on their side (manifold mounted) the windings around the core are exposed inside the can without the cooling
oil covering them, they run hot and can burn. This is more prvelant with the electronic ignitions because of the higher current capabilities.