i find this debatable. you can take metal straight from the store or garage and weld it and be fine as long as its not all rustyClean material and perfect prep work will guarantee strong welds.
fusion and penetration are what determines the strength, and in forms other then mig the tensile strength of the filler rod is a factor.
a sign of telling if you dont have the proper fusion and penetration is if the edges are rolled looking and dont flow fluidly into the base metal. its referred to as a cold weld