Since I work in the business (AMD) I can give a little insight. We do our own R&D and test fitting here in house in Ga before going into production and do our best to keep at least one of each model on hand to refer back to/test fit in the future if/when issues arrive. That alone makes a big difference between us and other guys, alot of the other companies don't keep vehicles on hand. We've put alot of effort in the last 10 years of making the best panels we can. We do source/resell stuff from other companies as well but anything that's branded AMD, we did the R&D and test fitting ourselves.
LMC develops their own parts as well, USA1 does the interior stuff themselves (not sure about the other stuff, I know they do resell some stuff from other places as well), Holley is just a big corporation now that's buying up smaller companies. As far as sheetmetal, I don't think they develop/make anything themselves, they're just a reseller.
There's not much I would tell you to stay away from, Sherman makes patch panels that are very crude in my experience. And also squarebody doors, Tong Yang's were the only ones on the market for a long time (crash parts from 30yrs ago). They're not terrible but they're not super nice and they're definitely lighter/thinner than OE. CHL's bed floor is a multi-piece unit that is often welded crooked (5 pieces making up the floor/skin) and causes fitment issues. Those are the big ones that stand out just off the top of my head.
