The MD-11's sales were limited not so much by safety questions per se, but mainly by the aircraft's inability to meet the original performance expected by the airlines. MD was also facing a cash crunch at the time and simply did'nt have the financial resources to build a better plane, even if they technically have the know-how to do so. It was unable to compete with the A330/A340s and the 777. Another reason for it's short production history is the fact that MD was taken over by Boeing in 1997, this meant it would be competing against Boeing's own products, so Boeing ended production in around 2001 I think.
Legitimate questions can be asked of MD-11's design such as the horizontal stablizer, etc. But the short production ofthe MD-11 was not driven by safety questions.