What is the major downside of using CO2 gas with GMAW?

Using carbon dioxide (CO2) gas with Gas Metal Arc Welding (GMAW) can indeed lead to excessive spatter, which is a significant downside. CO2, while being cost-effective and providing good penetration, tends to produce an unstable arc when compared to other shielding gases such as argon or argon mixtures. This instability is associated with more erratic molten metal transfer modes, resulting in increased splatter during the welding process. Excessive spatter not only creates a messy workspace but may also require additional cleanup and can compromise the overall quality of the weld, as it may form weld defects.

In contrast, other factors like weld strength or cost do not primarily hinge on the use of CO2. While spatter can directly impact the quality and appearance of the weld, and potentially lead to rework, the weld strength achieved with CO2 can still be adequate for many applications. Additionally, CO2 is a relatively inexpensive shielding gas, so the issue of increased cost doesn't apply here. Lastly, while reduced arc stability does seem relevant, the more pronounced consequence observed with CO2 is the excessive spatter resulting from the nature of metal transfer in the arc.