Theoretically weaving results in a higher heat input in the weld. A high heat input weld will have comparatively coarser grains than that welded with low heat input for the same material composition.
Coarser grains have compartively lower grain boundary lengths and consequently the toughness would be reduced.
However, limited weaving upto bead widths of 12 mm to 14 mm is acceptable in GMAW process for carbon steel applications.