Light dispersion is going to be the only issue where we have non reflector type tail lenses the light will just transmit straight through the lenses hence you will have very small bright round circles instead of the whole taillight lighting up
I had a look at taillight replacements and also came to the conclusion that today's LEDs would not work well, since there is no reflector in the taillight assembly. The LEDs I found online would look dim and spotty.
I tried a few sets in my tail light clusters and wasn't happy. The clusters of like 10-15 standard LEDs are not nearly as bright as an incandecent. They do make newer higher wattage LEDs that are supposed to be as bright, but are WAY overpriced and do not last as long in practice. (People are replacing $25-30 Luxeon LED's withing 6 months) I like the look of the instant on and off of LED too, but its not quite there yet unless its a factory installation in my opinion.