There are a lot of different opinions about the longevity of the ecoboost motors vs their complexity. Plenty of RV forums show nothing but love for the longevity of the engines. When I look under the hood of my 2.7, I too am reminded of the complexity of boosted motors, BUT, the power/performance/efficiency I’ve seen in mine has been worth it.
oil leaks at 120k? Totally believable, but I’m not sure I’ve owned a vehicle that wasn’t leaking somewhere by then.
I’ve also not been under the hood of a 3.5, but a turbo replacement on the 2.7 looks pretty accessible. Not that I’d expect them to need it by then unless you’re really working the truck. I sold my turbo Volvo with 160k miles on it and that engine ran and looked like it was new. Being that Ford owned Volvo for a while, I suspect there was solid technology sharing there.
i personally prefer the quicker snap of the 2.7 eco with the graphine block, but wouldn’t be afraid of owning a 3.5. Most of the issues I’ve known of are cooling line o-rings, leaky plastic oil pans, and the occasional timing guide wear - that one worries me more than the others - I run a heavier oil in mine due to that concern.