I have not used them, but I have read a few things that say the light from a standard incandescent bulb emits in a round pattern the shape of the glass and LEDs only emit light straight out. And the lenses and light housings that came on vehicles originally equipped with standard bulbs are designed to reflect and concentrate or spread the light pattern accordingly so if you use an aftermarket LED it won't look right in the light housing, but I can't confirm that. If you look at the retro LED bulbs there is usually a few different ones with different size and amounts of LEDs in different patterns for each bulb number. It's like a standard bayonette with a cylinder sticking out with some LEDs around the sides and some on the flat face.
They aren't cheap to experiment with from the truck stops, but I am sure they are way cheaper online.
And that's about all I know about them.