Rubberized undercoat works great if you apply it to a fairly clean and rust free surface. I did mine shortly after it was new and as long as you keep it maintained it will do well. But if it allows water to get under it due to scratches or chips, then it tends to hold moisture longer. And if not maintained it will eventually dry out and flake off.
I plan on putting a coat of POR on mine after I sandblast it and replace any rusted areas. I'm still debating whether to put undercoating or bedliner over that. I definitely wouldn't just put rubberized undercoating on bare metal or rusted metal. Use something that "encapsulates" the metal or rust first, to seal out moisture and air.