RPM's, torgue, one gear, drag, speed....

I think there are 2 main culprits.
1. Operator error
2. fuel quality
Combine the 2 and you have a recipe for failure.
And for anybody that think fuel doesn't play a huge part all I can say is they don't know squat about internal combustion engines and how they work and are designed.
As for operator error I think Steve hit it on the nail head.
WFO all the time is gonna take it's toll.

Two main killers of outboards: cylinder not getting the correct fuel/oil mixture and in the correct amount, and lack of coolant(overheating). In my opinion, both of these are a product of poor maintenance. You do the proper preventative maintenance and you should be able run the dog poo out of the thing for many years.

Every mechanic I ever asked about outboards told me that they are made to be ran wide open. I would rather buy a five year old boat with 500 hrs than one with 100 hrs. And I would like to see at least 10 percent of the hrs at wide open throttle. Cruising around in a big 2 stroke is like running a chainsaw at half throttle while cutting. It's not good for it. I religiously have a full service done every year and run the recommended oil and additives. Hours don't hurt the motor. Running them at the upper end of the rpm range doesn't hurt them either. Neglect and pinching pennies on service will hurt one faster than anything.

Thanks for the input from all. Probably do hear more of the problems with the popularity of forums like this and it could tend to overstate the numbers of blown motors.
One good point that I had not accounted for mentioned by Grout Scout on the Yamaha SHO recall perhaps giving the appearance of more failures than really exist.

Personally I have had all sizes of Mercury motors over the last 30 years with no real major problems. Mechanics have told me that lack of fuel/oil to certain cylinders with direct injection or high pressure fuel injection motors is an ongoing issue. Maybe four strokes have the edge on this as the cylinder continues to get oil lubrication even if the fuel supply is disrupted.

Exactly! The fuel injected 4-stroke was one of the main "must haves" when I was shopping for a newer boat earlier this year. They simply are more reliable.

Inspection, maintenance and warm up is the key.

All good points. Bad fuel leads to clogged injectors which leads to lean conditions and lack of lubrication which leads to over heating and burned pistons added to improper warm up and running WFO leads to Ka-boom. Not so much a problem with carbed engines as they are usually running a bit on the rich side anyway. Add in the ethanol which in too much % is going to lean the mixture way too lean and Ka-boom even quicker. Remember that 2 strokes don't have exhaust O2 sensors to read the exhaust gases to tell the computer to increase the duty cycle of the injectors to add more fuel when the exhaust gas oxygen content goes to AFR 18:1 or even higher. Ka-boom burned piston. Of course there are other factors like oil supply and such not working properly due to poor maintenance. Drive it like you stole it.
20 years from now a lot of the original four stroke outboards will still be running strong since the nature and design is much more forgiving for the lack of maintenance and bad fuel.

What happens when a cylinder gets too lean is similar to what happens when you press the oxygen lever on a cutting torch. Same amount of fuel and double the amount of oxygen and the flame front super heats and melts the metal. Add in lack of lubrication and things happen even quicker. Direct injected motors with air compressor may be the worst for rapid failure if one injector goes bad and the throttle is opened WFO.
You now have a cutting torch inside that cylinder with more and more oxygen being forced in and at some rapid pace it's going to melt something. Unless the engine is built with special materials an air fuel ratio of 14.7:1 is as lean as you would want it to run with 13.2 to 13.8 being even better. These engines are not designed to run at AFR above 14.7:1 and last very long. Especially a 2 stroke.
Modern 4 strokes in autos are using special valves, seats and piston alloys that can allow them to run at AFR 16:1 safely and not melt down and to meet EPA emissions standards. I would assume modern 4 stroke outboards are somewhat the same.