Yeah a LOT of the issues with the 6.0 and 6.4 were caused almost exclusively by Ford telling Navistar they wanted more power out of them. The de-rated engines put in larger trucks, from what I understand, weren't as plagued by issues.
There's also a lot to be said for care of them (I know you've said you care well for yours). There are a great many people who see DIESEL and think they can rag on it all day long... which wasn't the case for the 6.4 especially, the 6.0 could take a bit more but not much. If you threw a tune at EITHER of those with no supporting mods, the motor was toast, and sadly there were a LOT of people throwing tunes on them to make power because they were cheap...and then blew up.
I had a good 6.0, and one that gave me some issues (mostly with injectors). My dad has a 6.4 he bought new in '08 that he's STILL driving every day to this day. He needs a new box and the power mirrors don't work anymore, but he's never had a driveline issue yet.
Eh... to some degree Ford pushed it but what REALLY happened is that Navistar wanted to tout being the "first" emissions compliant diesel engine that didn't use urea so they started heavily publicly advertising it's planned capabilities. The VT365 (de-rated engine) had as many or more issues than the 6.0. We had entire bus fleets out of commission to the point that the state had to provide more money to the school systems to go out and buy Freightliners and Thomas buses to try and purge the fleet mid-year. Both have the 6.7 Cummins in them.
Ford got really antsy wanting to show the new 7.3 replacement to compete against Cummins future release of their new engine (the 6.7 at the time), and between the 2 PR teams pressuring, Navistar released the (VT365) 6.0 with less than a year of testing.
At the time, a lot of truck manufacturers were tinkering with SCR (Europe was way ahead) but no one had it nailed down. Mack had been testing out SCR on their E7 engines and Cummins was testing with their L9 engines. All required urea, and Navistar was convinced they could do it without the need for SCR.
When the 6.0 failed and Ford started screaming lawsuit, Navistar panicked and decided to release the 6.4 (which they had just started developing) with literally less than 200 hrs of testing. The Maxxforce 7 (6.4) was built right here in Huntsville, AL and I got to go out to the plant the day they rolled the first one off the line. The line wasn't even fully operational when they rolled out the 6.4. It never got finished.
And that is why Ford now produces their own engine. Ford approached Cummins, but Cummins has an exclusivity agreement with Ram for the light duty (2500/3500) market, so it isn't sold to anyone else for that purpose.
The Navistar plant here in Huntsville dropped the Maxxforce 5, 7, 9, and 10 engine lines in 2015. They knew it was coming, and their medium duties started getting offered with the 6.7 Cummins back in 2013. As of today, the plant only produces Class 8 engines for the heavy duty market.
