I dunno what specific aftermarket companies you're referring to (or even what body panels) .... but some A/M pieces can be built better, some worse.
Kinda like I bought a replacement hood for my Ranger and it was a cheap steel and flexed a little bit more that the stocker. Then, the A/M replacement bumper I bought for it was higher quality than the original.
What pieces were you looking at ... and from where?