Vfc e series gearboxes are identical to their higher end lines. Although I've never had one strip in any of my 5 vfcs, I've seen plenty of stripped vfc pistons, snapped tappets and unscrewed spring guides. That's not to say they're bad, but they are the most common points of failure.
I've never seen an opaque vfc piston, always clear or clear blue.
Hands down the best externals available, bodies and rails are anodized.
Guardians of Asgaard - KF32
VFC H&K 416C
VFC KAC SR16 JL Billet SBR
TM FN Five-seveN