With a couple of exceptions, the big AR manufacturers buy the raw AR15 forgings from the same place. These are solid aluminum pieces in the basic shape of an upper or lower receiver, with no machining done. Up to that point, there is no practical difference.
What comes next is the machining, and then the finishing, which is usually hard-coat anodizing (using black dye as part of the process), but may be something else depending on the manufacturer.
Hesse, on the other hand, uses mostly cast parts. These appear to be reverse-engineered, like they had casted someone else's receivers to make their molds, instead of being original molds. Nothing is crisp and straight like, say, an Oly casting, which is a high-quality casting.
Then, their machine work is BAD. Paul has a great example on his page. It seriously looks like something that *I* would make in my garage (having exactly ZERO machining experience or skill). Parts are ill-fitting and little is straight.
Finally, the finish is poor.
Their other guns are no better. I saw lots of Century HK91s built on their uppers that had rear sights that were horribly out of alignment, and they're welded, so it is very difficult to fix. My brother bought a FAL with their receiver and fire control parts. Nothing fits right, it's virtually impossible to take down without unscrewing the receivers, and the fire control parts had to be replaced.
Basically, they are either unable to make a quality product, or they simply don't care to. Either way, I recommend avoiding them, and any guns using their parts, completely.