If one follows the rules, a safety lever is a liability.
I disagree. If one follows "the rules" (meaning proper operation of the mechanism) a safety lever is never a liability, and while it might be if you're not following the rules, it also might be the thing that saves your butt from an AD, even if you aren't following "the rules".
Now, if you are speaking of the rules of a specific game, things might be different, but without detailed explanation, I don't see how...
The old boy that bought it usually was too busy talking witch interfered with his hearing.
ok, got it. He was properly instructed, but didn't pay attention.
When I was working in the Small arms shops in the Army, we called that "insufficent operator headspace".
An example of this, was a tread head who brought the coax MG from his tank, to my shop truck at the range, and said "it don't work".
I checked the gun, found nothing obviously wrong, and told him it was ok, go play... 3/4 of an hour later, he was back, with the gun and "it still don't work!"
I detail stripped it, inspected every piece there was NOTHING wrong with it, mechanically, I told him that, and sent him off again.
He was back again shortly after, with the same complaint (and no additional info, either). Frustrated, I geared up, gave him the gun and said "SHOW ME!"
Went to his tank (which was on the firing line) got in, he mounted the gun, loaded it INCORRECTLY, and when he hit the trigger it didn't fire, just jammed.
I moved him aside, cleared the gun, snapped off the damaged rounds from the end of the belt, loaded the gun correctly, and laid on the trigger firing the entire belt in one loong burst. Told him, "works fine for me!" and went back to my spades game in the truck.
I KNOW he had been trained how to do it correctly, but he wasn't doing it, so clearly he didn't listen to his instructions.
You have to be at least as smart as the gun, and its better if you are a little bit smarter.