Just wanted to chime in that I've been gaming exactly the same gun (SRC Gen3 Pro HK416) for the past 6 months or so and I've had no real issues. Only changes I made were the spring (M120) and very recently I switched to deans. I use a 9.6 NiMH nunchuck since it works great and I didn't find the minor advantage of a lipo to be worth the associated care/hassle.
I've put many thousands of rounds through this thing over that time (we play ~2x a month) - at least 10k if not more - and never had an issue.
Now maybe I'm just lucky, or maybe you're just unlucky
Given that the components in the gear box seem decent if not above average, I'd be curious exactly what the diagnosis was for what went wrong with your gun. You mentioned the piston; did it crack or strip or something?
I haven't used a ton of different brands of guns and parts (I have a classic army G36C in addition to the SRC HK416) so I certainly won't claim to defend any brand over another, but given the selection bias of forums it's usually good to present both sides
I'll also add that Ken has been great to speak with and buy from; as his post above demonstrates, he stands behind the stuff he sells.