I did have a ring land failure on a 240D, which has the same basic pistons, rods, etc, as the 300D, the 300D just one additional set.
The engine was taken apart because it was tired and the rear main seal was no longer touching the crankshaft anywhere. The engine had 280K miles, but I bought it at 203K, so it very well could have been started with ether like Steve mentioned.
I wouldn't think that broken ring lands are a common problem with this engine family. If someone said that, they may have seen ONE fail and made a determination that it was a weak point. It's impossible to make such determinations without tons of data. People like Steve and Benzmac have probably seen enough of these engines taken apart to have enough data to say whether or not this is a common weakness with these engines. There are several people who frequent this site who have seen a number of these engines taken apart. If you have no other response by now, I think it's safe to say it's not a common occurrence at all.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in