Much of the light emitted from a tail or stop light is transmitted by the chrome reflector behind the bulb. The shape of the reflector causes the lens to be filled with uniformly distibuted light. LED bulbs project the majority of their light in a cone or "V" pattern and the light would not go back to the reflector. This would show up as a bright spot within the lens, leaving the outer perimiter (and corner area) with low, or no lighting. To be truly effective, LED lights need to be designed as a unit with LEDs spaced to fill the lens area and have some facing the sides where the design is intended to light the side of the vehicle.
Another issue with LED signal bulbs is the fact that they do not draw enough current to activate many flasher systems. In certain applications, a special resistor could be required to make them blink.
Bob C