I've been using CO2 for welding for years witout any problems (including the the whole front end suspension setup on my truck).  Reason for me is price (monopoly actually on propper welding gases up here, only one company sells them, and only rents out the containers at a redicilus price).
Probably not as clean as using propper gas (or strong), but who cares when your welds are way longer and stronger than any factory weld on the truck anyway.