On the topic of plastic guns, it's not just flex you need to worry about, but how they actually construct the gun, and WARPING.
As I said in another thread, guns that are metal in real steel are specifically meant to be made out of metal.
Guns that are plastic in real steel, are specifically meant to be plastic.
If you try to make a metal gun out of plastic, you're gonna have a bad time.
The internals are stabilized inside the gearbox, but that's only a small part of the mechanism. You also need the gearbox to be very rigid against the hop chamber. There is a LOT of loose tolerances and flex points and wobble and lash in a regular AEG that you need to worry about even when you do have a metal body.
Systema is about the highest quality stock gun you'll come across, but you're limited to an M4 or MP5. Despite what anyone else says, the sole advantage of a ptw over a fully upgraded AEG is the rigidity and design of the barrel group. Everything else can be replicated in an AEG.
In the aeg world it's up to luck to find externals that fit together perfectly, there's a lot of shimming and tweaking involved which is why I switched to ptws. It's just much easier to get that top end performance.