Instead of just making comments "off the top of your head" perhaps some research is advised.
Plenty of OEM's use MIG, especially Kitplane companies. Any dedicated homebuilder with the proper quality equipment and training could duplicate the conditions and processes used in a typical Kitplane factory. It isn't a question of the process, only the skill and knowledge of the welder. And that applies to all welding methods.
As an inspector, I have been scanning for accident reports dealing with weld failure for 30 years. The only fatal weld failure incident I can recall was a crash that killed two EAA pilots in a Lycoming powered Wright Flyer replica a few years ago. In that case the NTSB provided detail photos of the failed TIG welds (lack of full penetration). I have not read or heard of any fatal MIG weld failures in any official NTSB report yet. Please report if you have anything.
It's sadly funny to read these comments. MIG can be ideal for many airframe parts. Maybe not all, but what is?
MIG is FAA approved, see AC 43.13