At the Aero show in Friedrichshafen, I talked to an engineer from Lycoming who used to travel the world for engine failure investigation. I questioned him on detonation and he said that they have only seen it when several things went wrong at the same time. One contributor is very high CHTs as peterh337 mentioned and apparently even worse is incorrect magneto timing.
I tried to find out how large the detonation margin is and while trying to be not specific (lawyers...), I understood that even for the non/not yet certified engines, it is virtually impossible to experience detonation with UL91.
His recipe was simple: make sure the magneto timing is right and do not lean above 75% HP. Below 75%, lean as you like.
Having no kerosene/diesel story, it is not a surprise that Lycoming is actively promoting UL91 and the like.