As I recall, none of the 4.0L CPS units were optical - I don't know what principle the RENIX sensors operated on, but the ChryCo units were Hall Effect. I believe the distributors were all Hall effect for the SYNC sensors.
The optical sensor has the advantage of not worrying about signal strength, but you either need a "reflective" sensor (emitter and sensor on the same side - alignment troubles and keep the targets clean) or a "transmissive" sensor (emitter opposite sensor, triggered by shutter wheel.) Either way, the sensor need to be kept clean, as does the emitter lense. This isn't as necessary with magnetic sensors - but magnetic sensors want replacing at intervals.
Also, when run in the presence of excessive heat and vibration, it's possible for the emitter and/or sensor lense to become misaligned (if the housing softens) and for the sensor to become useless - and want replacing. Then you have to align the thing properly - magnetic sensors are rather more tolerant of misalignment ("flying height," or distance above the trigger setup, is more critical than actual alignment.)
I've used optical trigger distributors before - Mallory has made them for a long while - but they do want to be kept rather clean all the time, and that's just a bit labour-intensive when the operating environment of a Jeep is taken into account.