I've had two 3.5's and excellent luck with both. My first was a 2011 XLT Supercrew with the 3.5 that I traded off with 184k miles on it. Passed the trade-in inspection with flying colors. Only issues ever were maintenance items (battery, brakes, tires, etc) and then around 175k a coolant fitting on the back of the head started leaking. About $350 later it was fine.
Traded it in on my 2020 F-150 XLT SuperCrew with the 3.5 in July of 2020. Got that truck with 53 miles on it, now it has 65,200. At 60k the dealer noticed some seepage on the rearend (pumpkin) so they opened it up, changed fluid and re-sealed it under warranty. Aside from a driver's heated seat that just went out in December in Nebraska it's been completely trouble free. Heck the stock Goodyear tires lasted almost 55k before I replaced them.
Can't say enough good about these trucks. I used to do a lot of commuting on the highway and it was nice to have all that power, good ride and absolutely quiet interior. My better half has a Lexus ES300h hybrid (2018) and my truck is damn near as quiet as that car. They both have gotten excellent fuel economy as long as you keep it reasonable speed wise or have favorable wind. My best in the 2020 was consistently 23.5mpg between Lincoln and Omaha doing 70 on the highway (NOT I-80). Best high speed was doing 85 north to south across Wyoming in the spring of 2022.
Now that I work from home I'm looking forward to likely keeping this one forever. I lucked out at the peak of covid finding the exact truck I wanted on the lot. I think the EcoBoost issues were overblown during the first gen - which did indeed have issues if you didn't drive it much or ever work it. Second gen is rock solid.