Mollies in general are extremely hardy and they are only fragile in soft water. Others like the balloon mollies are mass bred deformed (scoliosis) fish that you shouldn't expect to live.
Only P. lapatinna is a brackish water fish, P.sphenops and P. velifera (my favorite) are both freshwater species. If you keep a lapatinna in brackish water and regularly change the salinity up and down, the fish is practically bullet proof. I never had to deal with diseases in my brackish tanks, since relatively (key word) few bacteria and parasites survived the transition from freshwater to salt and back again (including nitrosomanas, Archaea are more resilient).
I have largely abandoned posting on forums (just read) but I used to respond to 50+ a day on the disease forums of Fishlore and still didn't cover it. I would disagree that saltwater is seeing more disease than freshwater. I would also point out that several of the medicines (antibiotics) are ineffectual in high PHs and others like Malachite Green (prominent anti-parasitic in freshwater) is extremely toxic at high PH levels.
40+ years Freshwater, NPTs, Hillstream, and brackish tanks
I look forward to building my monster saltwater tank (wall of my bar) in the near future, if my house ever gets finish being built.
I am going back into hiding.
Yes, I painted with broad brush strokes, but for most typical home aquarists, mollies are more delicate for the reasons I mentioned. I sold tens of thousands of them, and it wasn't until years later that I learned of their poor outcomes in most of the people's tanks. Fed algae, with some salt in the water, they do fine.
In my mind, I was just thinking - you give a beginner FW aquarist, a platy, a molly and a cherry barb, which two are going to be alive in 6 months?
Jay