Our experiences with the 6.4 Powerstroke indicate that the common failures are spun bearings and cracked pistons. The spun bearings come from diesel fuel contaminating the oil in the crankcase through the regeneration system and it is a catastrophic failure, meaning the engine can't be rebuilt. I believe that most of these failures can be prevented by servicing the engine regularly. (oil changes every 4-5,000 miles). Do not extend oil changes, diesel fuel is a lousy lubricant!
The cracked pistons is a different issue and can not be corrected. The 6.4 piston has a lip that extends too far out over the fuel bowl. The lip cools at a faster rate than the rest of the piston which causes some pistons to crack. We typically see cracked pistons starting at about 190,000 miles.
Turbo failures are also quite common on the 6.4 but both the turbo failures and the cracked piston failures can both be minimized by allowing the engine to idle for a few minutes before shutting it off. You should also keep that in mind when pulling into a fuel station. It may not be "politically correct" to the guy next to you in the Prius but if you leave your truck idling while fueling, that's about the same amount of time required for cool down so there is no reason to shut it off.
If you do find yourself needing a remanufactured 6.4 Powerstroke, Asheville Engine, Inc. is the place to go.
We Keep you Strokin'