Rich running will foul platinums as fast or faster than other plugs. Carbon fouling will lead to misfire, and low power, etc.
Iridium will work pretty much like platinum, but is more expensive and harder to work with -- will probably last longer, too since it is physically harder. Much cheaper than it used to be, since I have several gold/platinum/iridium crowns in my mouth now -- didn't cost that much (cheaper than porcelain).
Not all plugs will work well in all engines, and some will be more sensitive to actual operating conditions than others. I've got Bosch platinums (single electrode) in the 280 SE, and it runs fine. Not perfect, but I've not gotten the valve adjust done yet, nor is the mechanical advance in the distributor quite right. No knock, even on midgrade (this may change if the valves are tight), PLENTY of power, and 15-16 mpg mixed driving. They will stay in there!
I put plus 4s in the 300TE on the advise of my friend the MB trained mechanic, he uses them in all the cars he services. He says rough idle is a mixture/ignition problem, not the plugs.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!