I have experience with 2 Model 1 Sales kits....
On both kits, the gas keys were loose on both carriers. One was loose where after 100 rounds it was noticibly loose. The other was loose out of the box to the point where it rattled. The staking is poor.
One had an AR-15 carrier, the other had an "enhanced carrier" which has a shrouded firing pin area, and a little bit more meat at the rear.
Both LPK's were attrocious. Poor finish, crappy machining. They look pretty bad. After they got hit up with some brake cleaner, and the sludge they came with was removed, you could really see the poor quality of manufacture.
Handguards are the injected type, not the compression molded. So, if what I've heard is true, these will melt with enough rapid fire. They feel cheesy. But they work for most purposes.
Buttstocks were ok. Average quality. Like anything else. A2 stock came with a standard buffer in a package with dates going back to 1969. Must be some old surplus item (which is fine by me, at least its mil-spec). M4 style stock is ok too. No issues there.
Uppers are really nice. One is an A2, with the Cardinal Forge symbol. It's ok. The other has the Cerro Forge keyhole symbol and is a flattop. It is very nice. Well finished, well machined. Very good part.
The barrels are Shaws. They are very well parkerized. Well machined, very clean, very nice. Barrels are superb.
Charging handle is almost same quality as other brands, but the front hook part is indeed slightly shorter than "mil-spec" ones. This means you run the risk of the CH not grabbing onto that ledge of the carrier, and instead dragging on the gas key.
Overall, they aren't as bad as people make them out to be. They are after all "kit" guns and as with any kit, they sometimes require some work or tinkering to make right. These rifles function fine and have had no issues feeding or ejecting. They are accurate as well.
You get what you pay for is a rule that applies here. M1S uppers are actually pretty decent. It is their botls, carriers, and LPK's that suck.