Rubberized undercoatings are great for trapping moisture, causing more rust and rot. Something like an Eastwood Chassis Black or POR 15 would be a better choice.
I've used rubberized coatings, truck bed coatings, undercarriage paint, etc on lots of trucks.
If it goes on to un-rusted steel, its pretty good.
Put it on over rust and you've got further problems.
My 4th Gen Runner's undercarriage had so much rust I had to get rid of it. Toyota sent me pics of the frame with holes literally though and through. And it ran like a top when I traded it in. I loved that truck, even more than my current 5th Gen. Don't want that to happen to my current Runner. Is Ziebart any good? I'm not one to work on my own vehicle. Thanks.
Don't have experience with actual paint/permanent coatings, but I douse the underside of mine about twice per year with fluid film. It's great; repels water, seems to make rust bubble up a little, and doesn't collect all too much dust/dirt.