The biggest problem with putting LED bulbs in lights designed for filament bulbs is the positioning of the filament in relationship to the reflector.
A filament bulb has a single light source at a (roughly) known distance, so can be focused by the reflector and possibly modified then by the design of the forward lens, whereas an LED often has several light emitters, or in CREE units one very bright forward facing unit. In the former they are not in the focal length, and in the latter not even pointing the right way! Result is poor performance and often much light scatter.
The few cars that are now coming with LED headlights seem to have an actual lens on each emitter.