I have a 6.7 and so far it's a great engine, and surprisingly quick. Pulls awesome. I have a friend with the same engine and over 150k miles. Still going strong. Another buddy has a 2009 f350 dually with the 6.4. He had it deleted and tuned. He now gets better fuel mileage than mine, and runs like a bat out of heck. Before he deleted it, I could leave him in my dust, now it's reversed.
IF I were to acquire a truck with the 6.4, and IF it were in great condition, I would find a shop to delete, and bullet proof it.
My bud with the 6.4 runs a hotshot service to the oil fields in N. Dakota from south Louisiana, usually through Houston. He has over 450k miles on it now, almost all highway...still running strong. He tells me that when it starts giving him trouble, he will buy another 6.4 and drop in. He finally had to replace the ceiling liner and the drivers bucket seat at around 300k miles. I think he's on his 11th set of tires. He put a transmission in at around the same time. I recall him being somewhere in Oklahoma when it crashed on him pulling a load of pipe.
It's a good engine, but it needs a little tweaking.
2016 Ford F-350 Lariat CC, 4x4 SRW. B&W turnover with extender. 2015 CC Silverback 29RE, fitted with Reese Goose Box. Amateur Extra Class K5TS, 2016 nights camped 16