Monkeys running MIGs can make welds that look pretty good with little skill, but it is actually more difficult to make correct welds with MIG than TIG. In TIG the heat and filler metal are independently controlled these are not constant conditions they change all the time you are welding, but with MIG you just lay it down, regardless. A good MIG welder can do a lot to accommodate for the fact that everything is cold when he starts laying down a bead. He could if welding a trailer start on a side that has less structural load, work around the tube until he is welding at full heat in the loaded area, and then back around. Or he could make a tripple fillet and loose the cold start inside the fillet. In TIG every single bead can be melted in under perfect conditions. In small parts where it all counts you can lay out perfect welds, with perfect fusion of the materials.
Another advantage of quality TIG, is you can start the gas running before you add the heat, and let it run well after the weld is done, and still cooling. With other methods, or some TIGs even, you can't do that, and you will have a poor quality burned up weld, even if you get good penetration during the welding.
Not all MIG is the same, but most importantly a lot of MIG welding is done with fluxed wire, and no gas. This is very bad on small heat sensitive parts. There is a place for it. Bit anywhere you are thiking TIG, be absolutely sure that if you substitute MIG it is gas shielded MIG.