Yeah I can give you an earful on this subject since I finished a fairly extensive rebuild of my '71 Camaro in June. Aftermarket parts are not all the same! Some are junk that's way too thin, and ill fitting, and even the best brands can produce crap.
I was told to use certain suppliers, and that all their stuff was good. Some even advertise that their items are exact repros of factory parts, but that's just baloney. I found that there's only a few makers over in China doing all the body panels, and that ALL of them use new dies. The difference is the distributr. Some companies send their reps over to check everything before accepting the parts. Those companies end up providing a good product, because they do their own QC, while others negotiate a price and just buy what they get, then sell it. I know one company that has a huge number of cowl induction hoods for 2nd gen Camaros that are ALL off center, and they simply put them back on the shelf when someone returns them for defect, then resell it until it goes to someone who doesn't notice the 1/4" offcenter rib down the cowl.
When doing my Camaro I had to open up almost every hole in the frontend sheetmetal to allow parts to mate. Some holes were so far off that I couldn't open them up, and ended up cutting the brackets off and rewelding them in the correct alignment. My brackets from fenders to firewall had to both be chopped and reloacted. I also found that all the sheetmetal was much thinner than stock. So much so that the first time I closed my hood I dented it while pushing it down to latch! I had to drill holes in the hood and weld in a thin sheet behind that area above the latch to ensure it would be strong enough to hold when pushing it down.
I will say that most cast and machined parts I got (regardless of maker) fit perfectly, and looked exactly like originals. Interior repops were also of good quality with PUI, but not sure of all brands.
I found that National Parts Depot was the absolute best seller of quality parts. I spoke with a rep there, and they don't purchase anything without visiting the maker's plant and inspecting all parts before signing the check. Never had any issues with anything I bought from NPD.