One of this guys videos was shared earlier in the thread, but after doing a lot of watching, he definitely seems to have the most comprehensive testing in both number of products and duration, with realistic conditions. Seems like if preventing rust is your primary concern, especially on a used car, then woolwax, fluid film, or this new blaster surface shield are your best bet. Unless you want to do some pretty intensive prep work, the hard or fully drying undercoatings don't seem like a great option for a vehicle that isn't brand new.
Interesting. I may give that a try next season.