Rochester, NY is in the rust belt too, and I don't know that Mazdas around here are rusting any worse than Subarus. I'll try to be more observant and see if I can detect such a trend.
My (hopeful) solution for my Impreza is that I've enrolled in a car wash subscription at the local (reputable) drive-through car wash, which entitles me to one wash/day for a flat monthly fee. This wash includes a powerful under-chassis spray with fresh water. I use this service two or three times/week in the good weather, and more often in the winter.
The underside of my car looks immaculate almost all the time, and I'm hopeful that this will help keep rust in check. I won't know for sure until I see if my Impreza looks better than other Imprezas that are similar in age.