Originally Posted by MichaelW

When you are considering which is the easiest material to work with, you have take into account the build rate. Are you are making 5 frames or 50,000. Do you shape and machine your tubes by hand or with an industrial robot? Aluminium is soft and quick to machine so better suited to high-rate production.
Is the "difficulty" of welding aluminium really about the need for special kit or is there something harder about handling the torch than for steel? In a huge factory such as Giant, the welding equipment is as good as needed and their framebuilding jigs may be something a bit special for rapid production.