Dustin nailed it.
Heat treating, such as hardening and tempering, is not done until the parts are machined and fitted. A grade of steel suitable for the desired heat treating must be used, and then the heat treatment process itself must be done correctly. Then there are relatively simple ways to determine that the proper hardness was achieved. Springs can be fully flexed to see whether they break, or feel mushy, or take a set. Firing pins, hammer notches, trigger sear noses, etc. can be checked for sufficient hardness and toughness. It's called inspection and quality control.
Like Ted, I also own a Ugartechea Falcon, but mine has not been serviced or had heat treating redone. It continues to function flawlessly, but then again, it isn't used a lot. I got a great deal on it, and probably the biggest reason for that is that some Spanish gunmakers managed to ruin the reputation of Spanish guns through poor workmanship or complacency. I got it so cheap that I figured I could use it for trading stock on something else. But being a modern shotgun with 3" chambers, I decided to keep it to shoot up higher pressure ammo that I wouldn't use in my vintage or Damascus doubles.
Shotgunjones is also right... some manufacturers of guns, trucks, etc. simply continue to send junk out the door long after they are aware there is a problem. It's up to the consumer to do their homework, and to learn from others' mistakes. Sometimes, being first in line to buy a new product doesn't work out so well. With something like a new engine or firearm design, it might be a couple years before design errors begin to show up.
