For any of the vehicles mentioned, payload will probably be the limiting factor, not towing weight. The trailer tongue weight (not dry weight, but loaded for travel) added to the fuel, passengers and cargo in the towing vehicle will almost always hit the limit before you get to the max trailer weight that the the vehicle will drag down the road.
Also, the shorter the wheelbase of the TV, the less stable it will tend to be for towing - you could be sliding into a potential for a "tail wagging the dog" scenario. The wind forces on the TT from crosswinds and passing trucks can be more than the TV is able to stabilize. A good anti sway system with your weight distribution hitch will help, but isn't a guarantee if the TV is too light.
I'm a fan of the 3.5 Ecoboost, but I know nothing about the Flex, so I can't help there. For any unibody vehicle, the hitch capacity is a limiting factor too. The chassis is not as strong as in a vehicle with a full frame, and thus the hitch simply can't take as much load.