Biggest enemy of the 6.0. Powerstroke is the Ford Gold coolant. The silicates drop out of it, plug the oil cooler and egr coolers, cause flash boiling and hot spots which pushes out the head gaskets.
First thing you do with a 6.0 is flush the coolant and put a good extended life coolant in it. I ran Caterpillar coolant in mine.
They absolutely do not like tuning and power increases without studding. My first one was reliable but blew up right after I tuned it. I love the way they run, but there is a lot of things that can go wrong with them....and none of those problems are cheap. My F550 was studded, heavy duty coolers installed, blue spring kit, etc, etc.......and it still left me stranded because the injection pressure control regulator failed. Stupid little plastic piece that cost $1100 to replace at Ford dealer.
But I'd still buy one over ANY new DPF/SCR equipped diesel.