Which is fine for the parts of your car that gasoline touches.
In a direct injection engine, like the 3.5 Ecoboost, there is no gasoline in the intake runners as the gasoline is injected directly into the cylinders. When the intake valve opens at the top of the exhaust stroke, unburned fuel can revert past the intake valve and form carbon deposits on the back of the valve and in the intake port. Also, exhaust gas recirculation can introduce unburned fuel into the intake runners which forms carbon deposits in the runners and on the intake valve.
The 2nd gen 3.5 Ecoboost tries to address this by adding a port injector to spray gasoline on the back of the intake valve, but apparently this port injector runs very little of the time. The result is still carbon build-up in the intake runners and on the intake valves.
I agree that a lot of the cleaning services sold by dealerships are snake oil and way overpriced. If all they are doing is pouring something into the gas tank, you can do that at home with something like Chevron Techron. But, if they are using a system that injects a solvent into the intake tract, it might be worth it (though, I'm skeptical). Still, I think we are talking about a $25.00 (retail) can of solvent and a half hour of a technician's labor.