A SWG only frees you from daily dosing the pool with bleach. It won't relieve you of testing or brushing or vacuuming. If you get massive amounts of rainfall that cause the pool to overflow or require pumping down, you'll lose salt in the process. So just keep in mind that it's not set-it-and-forget-it. All you're avoiding is lugging jugs.
Again, if you get massive rainfall amounts in short periods of time, or you need to drain it down to winterize it, preplan for draining. If you have a main drain going in, plumb it separately from the skimmer so you can use your pool pump to get water down below the skimmer. And then have a Tee installed between the pump and the filter with a hose bib installed, or have a multiport valve on the filter that you can set to waste so you can empty it without having to buy a sump pump. Be aware that you won't be able to use the salty water on the garden. You'll need someplace to discharge where it won't kill everything.
That's about all I can think of for cons. I know if I wasn't constantly replacing water to reduce the CH buildup, I'd install a SWG.
