No doubt undercoating has improved over the years too, but for some time now, entire bodies have been dipped in a vat. (Not the final paint.) That has improved corrosion resistance greatly. I have scraped a lot of old undercoating off the bottom of older cars that solidified. Eventually, it pulls away allowing a pocket where moisture can be trapped. I have seen it. Back to my first sentence, I would hope things have improved. The key would be for undercoating to remain tacky. The last several vehicles I bothered with was to have them sprayed with used motor oil. (Used oil will creep more and find its way into small places.) You need to watch where you park for a few days after that. Probably can't find anyone to do it any more anyway because of EPA. You would just about have to DIY with a sprayer.