actual need for ballast resistor with electronic ignition?

the HEI does do some limiting itself
and they seem to work on anything from about 9 volts up to 16

HEI has a maximum dwell time set as part of the circuit.
its a dwell time appropriate for high rpm use with an HEI coil.
because its maximum dwell is set to a time period necessary to do enough sparks for an 8 cylinder at high RPM, they need that coil to be FULL in a short, in fact very short, period of time
so the module is built to switch 7 or 8 amps.... double the current into coil primary over a normal ignition, and uses a precisely specified coil so it can build a magnetic field much much faster. (this brings with it some other problems...)

BUT the basic thorn in the side of all of this is
anything like low or mid rpm use, the kind of stuff you do around town. is going to have the coil ON for too long and getting very very very hot
so they limit the dwell at low RPM i.e make that very short time for "coil ON" even shorter

and if there is no RPM it switches the coil off.

it does this based on the peak to peak voltage difference seen from the pickup

i.e the pickup in the dizzy acts as a timed switching trigger, to trigger sparks, and the size of its signal, peak to peak, is used to dictate dwell.
small signal= limit dwell
large signal means set dwell to maximum

the maximum dwell was calculated based, on rpm range, and on the coil characteristics and the much bigger current that the system operates at

so whilst an HEI module will work with any coil
you get the best outcomes with an HEI coil and potentially an HEI pickup

fluke/luck means our mopar pickup does a reasonable job. but we won't necessarily get the same level of dwell control from our modules as a chevy does, because we are bodgeing two systems together

so when you get someone who says their HEI can't maintain spark at 5000 rpm and therefore HEI is crap... maybe they just have a mismatch between the module and the trigger, its probably running in dwell control mode even at high rpm, and its the trigger that's crap. stick one of those tiny chinese button magnets to the back of your trigger head and see if it gets better..... bigger magnetic field at the pickup will make the signal bigger....

the other thing it does is to switch into a kind of limp mode if the primary resistance of the coil is way way too much. it expects 0.7 ohm or thereabouts, whack in a 3 ohm 12 volt coil off a VW beetle and get quite poor performance... use a blaster 0.7 or 1.? ohm or similar and get OK performance.
The lucas DLB198 electronic coil was designed to be used with HEI on a jaguar. They are OK I have had decent service from them, but i prefer the laminated core coil off a mid 70s to early 80s chevy or ford i 6 truck.

Dave