Film and optics
I am an engineer for an aerospace transparency manufacturer. One issue not mentioned here that would be a big challenge is achieving satisfactory optics with an applied film. Getting perfect optics with just formed acrylic or polycarbonate is hard enough. Trying to apply a film evenly over a contoured surface would be ridicously hard. Car windshields and many fixed wing aircraft have very simple curvature. Most helicopters have some what spherical shapes. They do not lend them selves to getting something to lay flat on them. Just getting protective film on them for shipping can be tricky so we spray them with peelable latex.
Also, polycarbonate windshields have a silicone based hard coat on them to prevent yellowing, crazing and scratches. It will not accept something you try to stick on it. Some acrylic is coated as well, but not as often.