Paint-like rust inhibitors can trap rust. They don't wick into small cracks like a thin oil and once they cure water and salt can get behind them. Some people have luck but I've seen enough failures to make me wary. What I did to mine in the link, fair amount of work but less than what you're describing here, and a lifetime coating other than touching up the bottom of my diffs every 3 years or so. Would still wire brush the loose scale - Ultimate oil undercoating guide (DIY Waxoyl)
Agreed! Wool Wax, Fluid Film and from what I can tell your Ultimate oil all work off the same principal. They are nota paint like coating (like POR 15). They are organic, self healing and never cure into a rubber like coating thus hastening the rust problem. I bought a used Tundra awhile back that had Zeibart undercoating and there were a lot of areas where the undercoating looked like there were big bubbles of rust under the coating. The first owner took it in every year for a touch up but I think it exacerbated the rust issue in that 2011 Tundra frame.