I was very happy with my old KWA M4S, and I've been very happy with my KWA SR10.
I haven'd had any trouble with either of them.
Any company can put out a lemon every now and then, and it is entirely reasonable that they won't honor a warranty after someone other than one of their own techs has taken the gearbox apart.
You are going to find this with ANY product. If your 360 RRODs on you and you try to fix it yourself before sending it in under warranty, guess what? you are simply shit out of luck.