Anytime I would weld critical stuff cages, links, control arms, etc and etc it was only Tig never Mig. I haven't worked on race cars for last 10 or so years but if I started back I would only use Tig.
If you weld Mig don't assume just because the welds "look" good that they are strong.
TIG is freakin art, and it's pretty pricey to get set up which is why many amateurs/novices would "graduate" to it. My experiences with MIG welds that looked good but broke had good penetration on one piece but not the other typical mistake for me was an inside corner. It looked liked it had good penetration and it did halfway. When working with multiple thicknesses it's an easy mistake to make.