Assuming one can produce proper welds with adequate penetration, sufficient accuracy, and no undercutting, etc... The usual stuff, then the main problem with MIG is cold starts. The wire that initially feeds and is consumed is colder than further down the joint, and while the general weld may be fine the starts may not be. There can be similar problems when incorporating spot welds. Since tubing in general and small tubing in particular requires quite a few stages to weld, these can be specific problems.
Also, there is a lot of different MIG. A gas shielded MIG process is way better than a cheapo box feeding fluxed wire.