[120mm]

Slightly less snotty response, but still relevant. MIG is perfectly acceptable for welding things that don't fall out of the sky if they fail. It's the industry standard for many of the Cobra kit manufacturers, in fact.

TIG is used by some industries for processes because it makes sense for their business model, or it has some capabilities which benefit their process.

O/A is how to go for ultimate strength in anything with a tube chassis. Plus it is compatible with alloy welding. It's main drawback is that it is a legacy process that is expensive and puts a lot of heat into the piece being worked, which results in dimensional instability (which then needs to be addressed, which takes time and skill).

The difference I've seen in amateur versus professionals vis-a-vis welding is that amateurs over-emphasize one process as the "one and only acceptable" process out of the fear of getting it wrong.

I wouldn't have a problem driving or riding in a MIG welded chassis cobra replica, provided the welds were done right. In fact, I already have, once. In a fairly high mileage machine that was holding up quite well.