Just to make it perfectly clear all of my comments regarding the probability of failures reflect the situation for Lycoming/Continental/Franklin engines in certified aircraft, or the engine that is in what I think are the aircraft that the vast majority of the readers of this forum flying.
My comments do not apply to ultralight aircraft, especially those with 2 stroke engines as these types seem to have many more instances of mechanically induced engine failures then traditional certified engines. I have had 4 students of mine go on to fly 2 stroke powered ultralights and every single one has had a total in flight engine failure that was not caused by something they did.
For this reason when asked I do not recommend that my students fly 2 stroke powered aircraft and if they do they make a point of always being in gliding distance of a survivable landing area.