To my knowledge, nobody's had an engine failure in flight due to mechanical problems with the engine itself.  Most of the problems people have had with rotaries have been with cooling or fuel delivery, which I would blame on these systems having to be designed from scratch, often by the builder and on the fly.  It's no surprise this will improve as the knowledge base grows and best practices become more well known, just like rotary car tuners who once blew motors every 20,000 miles ended up being able to make the RX-7 more reliable than it was new.