Hey neighbor! I'm just outside of East Aurora. They say FF or WW can swell natural rubber products, but I don't know of any natural rubber on a vehicle these days. It's safe and recommended for electrical connections, wiring, soft parts, etc. Like you say, it does create a dirty, greasy undercarriage, but I'll take that over rust any day.
I too have and continue to use both WW and FF. I've found that if you are doing the application on an ~80degF day, both products ssem to thin, cover and flow the same. I think I've mentioned it in this thread, but to me, the main difference is WW smells much nicer than FF.
WW does seem to hang on to exposed surfaces better. They both wash off areas that see heavy road blast.