In my honest opinion, there are no cons. It makes the pool nearly maintenance free as far as chemicals. I do nothing but skim every day or two, and vacuum pool bottom every couple of weeks.
As far as the rusting of the metal parts, that's BS. There are a number of threads on this site that go into detail. What causes the rust is the metal parts that make contact with each other and rub the finish off. I've been using SWG for many years now and it's the only reason I still have a pool. If it were up to me to maintain the water chemistry, I'd have an algae filled mess.
A good Hayward T-Cell system, and a good sand filter is all you need.