In my experiences the biggest problem is that they are usually used in these cheap nasty welders, which suffer from the two problems i mentioned before. Irregular wire speed control and inaccurate voltage settings mean a poor setup for trying to get any decent weld out of them. The settings / controls on the MOG welders have to be pretty accurate to get a good weld, more so than regualr mig. They are prone to porosity, if the flux isnt burnt off before the weld cools. But i guess this once again stems from cheap shitty welders we see on ebay etc.