Right now I only have 2 EMs. They are both Bally of similar vintage, Bon Voyage 1974 and Monte Carlo 1973. They behave very differently as far as the LED insert flickering. In the BV, you can just barely see it. I didn't even notice it at first. On the MC it is the exact opposite - very noticeable.

You could get away with the regular LEDs in the BV, just barely. No way in the MC, it is too annoying during game play. As you said the non-ghosting LEDs get rid of the flicker, I did test this. But as an alternative I have been soldering some 1/4 W 470 Ohm resistors across the lamp sockets. That trick seems to work for the EM like it does in the SS machines, at least for my testing. Its more of a pain but cheaper

What I am not sure of that would take more testing: Is it just the nature of each of these 2 machines to behave the way that they do? Is it the way the wire looms are organized and routed? Or is it some other difference in the machine? And, would it be the same for every BV and MC, or is it a situation where each machine is different on its own for whatever reasons? Lots of unanswered questions.

Just to make clear: I don't see any actual flicker in the sense of a constant flicker like 60 Hz. And the GI is fine with the LEDs. The problem is with insert lights with LEDs when they are not lit. They will slightly light up when various things happen on the playfield, like the ball hitting the pop bumpers, various targets, etc.,in coordination with those events. Some sort of induced current in the wires caused by the solenoids energizing I presume.