The expedition does not have a car chassis, it is a ladder frame body on frame SUV with independent rear suspension. Towing is only problematic with improperly equipped for towing, I. E. A WDH not setup correctly, wrong tires, improper loading, etc. Look in the towing section here and see what others are towing with expeditions, both the old 5.4 and ecoboost versions. The higher trim vehicle you purchase, the lower the payload you roll have. Do not trust the dealer, trust the yellow sticker on the truck door frame. Dealers will quote you maximum figures for the vehicle platform, not necessarily the one you are looking at buying.
My 17XLT EL 4x4 with HD tow pulls my rv through the mountains of Idaho without any issues. My trailer weighs 6300 pounds. I would feel comfortable pulling up to 8k with good WDH and LT tires. I use a husky centerline TS hitch with 1k pound bars and am stable at speeds up to 75mph. You won't find another full size SUV that will pull like an ecoboost expedition except maybe for a 6.2L Yukon xl but interior space and comfort is greater in the expedition due to the independant rear suspension. A 17 will have sync 3 and limited slip differential. Add a powertrain warranty to cover the turbos and motor and you will have a sound reliable towing platform. My wife daily drives our EL and has no trouble, it turns sharper than a pickup and is shorter.
My 17XLT EL 4x4 with HD tow pulls my rv through the mountains of Idaho without any issues. My trailer weighs 6300 pounds. I would feel comfortable pulling up to 8k with good WDH and LT tires. I use a husky centerline TS hitch with 1k pound bars and am stable at speeds up to 75mph. You won't find another full size SUV that will pull like an ecoboost expedition except maybe for a 6.2L Yukon xl but interior space and comfort is greater in the expedition due to the independant rear suspension. A 17 will have sync 3 and limited slip differential. Add a powertrain warranty to cover the turbos and motor and you will have a sound reliable towing platform. My wife daily drives our EL and has no trouble, it turns sharper than a pickup and is shorter.