Any process is capable of making an acceptable joint for a motorcycle frame. Stick is not used because of the thin metals involved. MIG has been used since its conception to join motorcycle frames on production motorcycles. TIG has been reserved for the custom motorcycle where the appearance of the weld is important. The use of TIG has nothing to do with safety. Of course, commercial frames are more heavily gussited than custom frames so maybe they can't get the strength they need with MIG? Oxy Acetylene used to be the most used process by the hobbiest for welding up motorcycle frames and roll cages. It is still prefered for chrome moly by the aircraft welder. Torch brazing also used to be used for chrome moly motorycle and bicycle frames. There are lots of acceptable practices for joining motorcycle frames. My rule of thumb is, if you don't have the experience or knowledge to know which process to youse, you shouldn't be welding up frames.
