I think the big part of the ban is the poor efficacy. But not saying the efficacy of PROPERLY operated merc's is poor, but the efficacy of AVERAGE maintained ones is really bad. All this, because the way merc's behave at end of their life: They are only a bit dimmer. And you might explain thousand times, then after 4yrs they have to be relamped, but you get an answer "Why, it still work". So 80% of private merc's is so blackened, then they are barely visible alone... Funny thing is, then the same case is for now promoted LED's... :-)
This wasn't totally the case in the 70s.....70s MV would stay bright for like 15 years or so due to the Lifeguard feature in Westy lamps, and other brands using their own feature name brand..