Whoever told you the 6.4 was bulletproof was blowing more smoke than a bro truck on a race tune.
The 6.0, believe it or not, is a really great engine, once the problems that crop up are addressed. The problems remained consistent, but the time to failure was the variable, depending on how much fuel was applied, and cylinder pressure wasincreased.
The 6.7 is clean sheet, and the only one like it. Intake is on the low side of the head, exhaust up top, turbo in the valley with short runners. That fixed the exhaust bellows and up pipe problems in short order. The oil system was fixed, the cooler is outside the engine. The cooling system was fixed... a split design to handle the oil and turbo cooling differently than the heads.
Yes, there are some problems, DEF tanks being one, the fuel system being two, IF exposed to water. I'm willing to bet the owners with problems drove the crap out of them, little maintenance, didn't drain the water separator, didn't add anything to the fuel to deal with water.
A FASS system, a DPF friendly tune from DPTuner, and regular maintenance can and will make the 6.7 as close to bulletproof as you can get. Even regular, diligent maintenance stock can get impressive numbers.
You can look and find DuraMax and Cummins problems as well. But you wont find too many mentions like this.