Even a fully forged engine can be destroyed with nothing more than a tune or a bad mix of fuel. The 3.5 EcoBoost is by no means a "weak' engine. The factory 3.5 EcoBoost F-150 is able to pull Davis Dam, with a trailer, at full GCWR, at Wide Open Throttle in 100+ degree temperatures repeatedly without failure. It's a 6 bolt main engine with an additional main bearing support brace and piston oil cooling jets. I know you don't want to hear this, but the only variable is your tune.
What is important to remember is that these are 6,000lb trucks with the aerodynamics of a brick; the engine is under much more stress that it would be in a car application. When manufacturers design engines, their durability targets are based on application, power, output, GVWR, etc. They don't figure in aftermarket modifications; that is an unknown. During testing, there is no engineer that says "everything passed...but we need extra budget to upgrade the rods in case someone installs an aftermarket tune and increases boost". That is true for all manufacturers; the days the factory overbuilt engines are over unless you're talking niche applications.
I would recommend bringing it to the dealer and be honest about the mods. That will get you a lot further than trying to make it appear stock and having them find out later. The issue could be as simple as a failed high-pressure fuel pump roller tappet or as bad as a melted piston; but better to let them know up front what they should be looking for. If you bought the truck new from the same dealer you're having it serviced at and have the maintenance plan through them, they are much more likely to work with you because you're a customer. That may seem trivial, but in my personal experience that can be the difference between helping you out with the cost (hours, parts markup) vs. telling you to pound sand.