I've yet to see ANY aftermarket body panels that were great.
I think it's a matter of the cars not being super consistent and the factory developing ways to slap em together more than it is the aftermarket producing crap (although they certainly do that too).
Look at roofs on new cars. No body putty. A couple black strips and you're done. I had that idea ten years before they were doing it! The closest our cars came to that was a vinyl top, unless you were getting a window plug at creative industries.