One of the reasons people favour tig for structural welds is temp control, mig doesn’t give you that level of control, mig welds tend get somewhat hotter and be harder once cool, so can be less tough, ie brittle, tempering post weld with a flame and tempilaq or temp crayons at about 650C will result in a more ductile joint, likewise pre heating the area around the weld with a flame can also help as the weld doesn’t cool as quickly, so won’t end up as hard, I don’t know what measures Yamaha took to temper and or stress relieve their frames when they were made, but they do seem quite tough, don’t hear of many failures.