As far as the welds themselves go, some are better suited to particular tasks than others. It's awfully difficult to weld 11ga pipe with an ARC welder without burning thru. Frame rails on a regular rectangular frame are only .083 wall and that's even thinner than the 11ga. MIG or TIG would be the preferred method of welding on that material also. Now around the rear end housing 4link plates or ladder bar mounts I always use an ARC welder to make sure I get good penitration there. Yup, I guess the bottom line is any weld be it TIG, MIG, Osy Fuel or Stick is only as good as the person doing it!
When I built my Studebaker it had been a few years since I had done any MIG welding to amount to anything so I got a friend who does it all the time to do the finish welding after I tacked it all together. Welding is just like a lot of other stuff, you have to do it on a regular basis to be good at it.