What is being said here flies in the face of everything that has been learned within the windsurfing community for the last 30 years.
When Dacron was used, the rig was easy to overpower and had a very narrow wind range…the Dacron/Mylar laminate was better…then battens were added which helped increase the range, then camber inducers to force more shape into the sail. All these things helped, but when the sail manufactures started to use Mylar film and scrim/Mylar sandwich things really started to happen…all the sudden one sail could cover the wind range of 2-4 Dacron old school sails.
The designer could then design the shape into the sail and be sure it would keep its shape in a wide wind range, if set on the correct mast and used with low flex carbon booms…
The sails became much more responsive to tuning…if you set the down haul or out haul a certain place, it stayed there until you changed it…the wild fluctuation of center of effort in the sail was now gone…as the wind picked up by proper adjustment of the down haul and out haul the center of effort could now be brought down lower and moved forward on the sail making it more controllable, with the head bleeding off excess power that would slam a sailor with similar size old school sail under equal conditions.

You can’t keep the design foil in as wide of wind conditions if you have a fabric that is constantly changing shape. Dacron is generally cheaper, and more resilient to rough handling, although I have had just as many Dacron/Mylar windsurfing sails delaminate as I have Mylar, mylar/scrim/mylar sails.

Bob