Based on what I've seen/read/experienced the past 7 years.
Tuning problems are a major problem. However, most people never get to the point of tuning, so I would put that last.
Fuel problems. If anything, our fuel problems are the cars run too rich. Sure every now and then a fuel pump fails, or an injector seizes... someone leans out an pops. Has this happened a lot though? I think the problems occur during normal driving over a period of time and are "discovered" via diagnosis. Ultimately these failures initially prevent an overall engine failure.
Overheat and overboost I'd say are nearly on the same level. Overheating will kill ANY engine. It may do it slowly, or instantly, but it's death no matter what. However, people do drive around with engines drinking coolant. Hey, if you keep filling up it will still run right?
Overboost. That will really do it. Why overboost occurs is its own topic. But what it does will break an apex seal and the car is rendered useless.
I'm not an expert and not as experienced as many people here, but this is my list and I hope others get something out of it.