one of the main problems of the WA M4 design is the bolt catch.
it's not so much that it doesn't work, since it does, but more along the lines of how it has been designed to work.
the second major issue is the bolt and how it's unsupported during operation.
as the carrier cycles and the bolt moves out from the carrier from the blowback pressure, the bolt becomes free to wobble around enough that some times both the back and front end of the bolt gas inlet strikes the bolt catch during recoil and battery. this doesn't seem like a huge issue at first but this can cause quite a bit of damage to the bolt gas inlet.
being that the bolt gas inlet and the magazine gas outlet gasket are flat and horizontal, they rub against each other as they cycle. so what right? well, if you have a plastic bolt it's not such a big deal since it's not really hard, but if you use metal bolt the inlet will become rough and will have some sharp jagged edges on it. this ends up chewing up the rubber on the magazine gas outlet, and now you have a crappy seal between the bolt and mag.
this brings up the next problem. since the bolt inlet and the magazine outlet are horizontal, any wobble of the magazine in the magwell will cause a misalignment in the seal and waste a lot of gas. this is also the reason why some WA-G&P-AGM owners are having instances where they pull the trigger and the mag dumbs all of it's gas.
there are other smaller issues, but nothing really worth talking about.
one of the current problems seems to have been addressed by VFC and their GBBR which is based off the WA design. they seem to have solved the boltcatch problem, which i really like how they redesigned it. the section that contacts the bolt now only moves vertically and no longer rotates on an axis like the WA design. this eliminates that problem of the bolt inlet striking the boltcatch.
also form what i have read about the VFC redesign, it would also appear that they have redesigned the top of the mag (the gas outlet) and the inlet on the bolt. this might be why they stated that the VFC GBBR's will not be compatible with WA style mags.
from and engineer's perspective i would take a guess as to what they changed on the mag and bolt. i would assume that they change the seal configuration from a flat horizontal style to an angled and or curved shape seal. this would not only improve the seal contact between the bolt and mag, but also prevent the bolt inlet and mag outlet from rubbing on each other. this means that the seal would last longer and it would also help push the mag down and forward, reducing mag rotation in the magwell.
i'm really looking forward to the release of the VFC GBBR's, especially if they have indeed fixed the problems that were designed into the WA system.