Boeing has a presentation (PDF) that shows accident rates by type (p. 23). Note the big spike for the MD-11. Only the 707 (1957), DC-8 (1958), and Fokker F-28 (1967) have worse rates.

To swap the tail fin engine on a DC-10/MD-11, you have to disassemble a large part of the tail because the engine hangs over it which is the labor issue.
See the illustration at the bottom of this page.

Center-mounted tail engines don't have this issue as they can be dropped straight down, but the S-duct introduces some complexities.

Ultimately, a twin with ETOPS is cheaper and not really any more dangerous.