As I and others have said several times on this forum L is not meant for that purpose. Ford says this. The brake pedal can apply all the regenerative potential the car can offer in a progressive fashion. Driving in L all the time means you loose the "semi-coast" situation when you lift off the gas pedal and have only very mild regen. In L the car will apply maximum regen breaking unless pressing the accelerator overrides this and switches the motor to acceleration mode. So you are either accelerating or breaking heavily. This "binary" driving style is not the most efficient as there are losses involved in the regen process. So it is better to drive smoothly and minimise braking. In addition using L does not operate the brake lights so could cause issues for someone following.
I have a C-Max Energi which has the same L feature and operates in the same way when driving in EV mode. I find that it is difficult to gauge using L for example, coming to a stop at an intersection on a 90km/h road. Either L it is too heavy and requires taking it out of L / using the accelerator again or it is insufficient which means hitting the brake pedal as well. Better to brake gradually using the brake pedal and start braking earlier, so as to use regen braking and not the friction brakes. This is what the brake coach is for, to assist with getting a feel for how to achieve 100% regen. That means 100% of the energy the car is capable of capturing, not 100% of the energy of motion (due to system losses).
The way to get maximum range, if that is what you want, is to employ hypermiling techniques which many will be uncomfortable with as it involves using a mix of D, N and L.