Your suggestion that "marginal grounds" caused more problems with LEDs is exactly the situation that I have on my 1954 Chevrolet 6v Suburban. The original bulbs had gotten so dim that friends told me not to drive at night. I switched to 6v LEDs and the ones that work are much brighter than any of the original 6v bulbs. But, some of the LEDs do not iluminate, or, they are intermittent. I attribute this to bad grounds, resulting from the excellent epoxy primer and urethane top-coat paint. Bad grounds from good primer/paint.

On the 1954 pickup that is ready for finish painting, I will ream the lighting-fixture screw holes using "wire brushes/bits" made to clean screw holes. I will eliminate future rust in those holes by treating them with anti-oxidant stainless-steel grease.