Ford made a LOT of the 6.0 and 6.4 Superduty trucks. I would venture a guess that 90%+ had no major problems and there are plenty of people that don't frequent internet forums and have no idea that anything could be wrong with the truck.
The problem is that the relatively few trucks that have problems tend to have a MAJOR problem. My friend has a 6.0 in his F250. It was a great truck... until he had problems with the top end of is engine. He had to spend a good amount of money fixing it... but hopefully it will last a good while longer.
Chances are that you could buy that 6.0 and have many trouble free years and miles. The only problem is that you might be one of the unfortunate few that gets saddled with a major repair expense. For some people its worth the risk, for others it is not.