I for one have owned 2 6.0's. One excursion and one f350. the 05 excursion has over 180K miles, never an issue, but minor things, Alt, batt, and regular maintenance.
My F350, has 90k miles on it, original alt, new batteries, regular maint, but I did just have my Fuel Pressure sensor flaking out.
NOW, as other posters have said, you can get a good tech, and you can get a bad or lazy tech.
I am fortunate that I bought my truck from Larry Miller and recently moved to Salt Lake where they have two Ford Dealers by Larry Miller. I have a warranty on my truck still.
But my issues with it was scary twice. First time, it lurched at a stop light and my foot wasn't on the brake that hard, felt like I got rear ended by a Honda. The truck jumped and powered through my light braking and went a good 5 feet. I learned to keep my foot on the brake.
The second thing it did was die just cruising down the freeway, I kicked it in neutral and the engine kicked back in on it's own.
Then the erratic RPM up and down by about 200rpm started. I took it to the first dealer and it was doing the RPM hunt but they swore up and down it was normal due to AC cycling. Now I work on Multi-million dollar machines, so I'm no dummy. I recorded it with my phone and took it Provo to have that dealer look at it. That day it didn't do it, but I told the tech it is the Fuel Pressure sensor and I showed him the video. He asked why I thought it was the sensor and I told him I googled it. He laughed and I told him to just plug in, drive the truck and monitor the signal. Sure enough, he said after about ten minutes it started acting up and the signal was dropping out. They replaced the part and the truck has been running awesome. Has a bit more power and my tail pipe smoke went down.
Now just so we are all clear here. My 6.0 trucks are all STOCK except a small lift and tires. No tuners, no exhaust kits and no intake kits. The 6.0L is already tuned pretty high. If you want a truck with little to no issues and are looking at used, then find one that is near stock except for cosmetics. Also, if you buy at a dealer, pay the 1500 for the extended warranty, the first time something goes wrong, it will be a minimum of $500 to have it worked on.
I normally work on all my own stuff, but I paid for a warranty and so far that one repair was $700.00 and mostly labor. So that warranty is already paying off.
Next, you have to keep up on the Coolant changes, EGR cleaning, EGR cooler cleaning, and oil cooler cleaning. It is best to plan a day during the spring to just drain the coolant and oil. Pull the ERG cooler and Oil cooler, soak them, clean them and re-install them. Also clean the EGR valve at the same time. If you have the extra $50.00 change the fuel filters while you are at it.
Also, call around to places like 4wheel parts or any off road shop in your area, they will tell you who is good and bad about working on diesels. Diesels are not hard to work on, and I am not sure why people are so scare of them. But the parts do cost more. However, I know that most places, they are not real good at troubleshooting diesel truck problems and will just throw parts at it. I have a hard time trusting people working on my vehicles, especially if they tell me it's a turbo issue.
My buddy has am 08 cummins, he bought it on a Friday and it spent the next two weeks in the shop under warranty. Basically he was going home from work and all of a sudden the truck shut down. He called me and I told him it might be in regen mode and to just drive it. It never recovered and dodge spent 15k in parts and labor trying to fix that truck. My buddy called a local Diesel shop and had it taken from the dealer to the cummins shop. Now where we live, a truck over 8k gvwr is not required to have emissions, so the shop pulled it off, changed the exhaust and added a bully dog to it, cost my buddy $3500, which he was going to do anyway, but that truck hasn't had a problem since. So it just isn't Fords that have issues. The 2004 duramax is known to have injectors fail and fill the cylinders with fuel causing bent rods. Go figure.