Generally, it is a poor workman who blames his tools, however, in the case of welders, a good weldeing machine is a lot easier to get a quality weld out of.
There are so many variables when welding - heat, wire speed, is the metal properly cleaned, if thick - too thick for a single pass...In short - it is up to the person welding to make a good weld.
The MM135 was most assuredly NOT the weak link in that Baja Bug problem. If the actual weld failed, that is a result of bad welding prep or technique. If the metal around the weld failed, but not the weld itself, the it may have gotten too hot or been poor quality metal.