Sources?
Carbon buildup on valves is a common thing to all direct injected engines; it's not something that is a problem for ONLY ecoboost engines. Ford has corrected that problem with the new dual injection system on the new ecoboost, so I'm going to say that on that count you are wrong.
What is it you consider to be "basic replacement parts"? Starters, alternators, air filters, oil filters, fuel filters, water pumps are all the same prices. I'd like to see the part that is more expensive for an ecoboost than the 5L... There are some expensive parts that aren't on the 5L, such as turbos. I guess the exhaust manifolds might cost more, but I don't really consider them to be "basic replacement parts". I think you have a weird idea of what a basic replacement part is, or are wrong on this count too.
Funny thing, bought my 2015 F150 with 3.5L ecoboost in March 2016. I've got 57,xxx+ miles on it and have had zero issues. All I do is add gasoline every 600-700 miles, do an oil change every 5000 miles, and change the air filter every 10,000 miles. No fixes or repairs needed on a flawless truck...