my observations on the 6.0L, from having one, and knowing many people who do.
for the record, my 6.0L has a programmer, has an aftermarket egr cooler, factory replacement oil cooler, gaskets and studs. original turbo and original injectors (it needs 2 injectors now)
A lot of the original 6.0L problems started because the dealers were misdiagnosing problems and throwing parts at them. Ford didnt offer enough training/documentation to the dealers to help them do their jobs properly.
also, the EGR systems on these trucks, cummins included, really needs the truck to be WORKED. daily driving them at 5% of their capability, is really not good for them. you get a lot of build up and basic problems start.
the truck was designed to be worked, i know mine runs better when its hooked to my 10K lb enclosed trailer.
Also, people started modding them with programmers, aftermarket turbos etc, but not monitoring their gauges or knowing enough to do it safely. Of course increasing your boost pressure beyond what is factory is going to cause more pressure in the cylinder, hence heads lifting.
but this is just an opinion from a guy who is generally happy with his 6.0L