Is bed liner better than something like 3M rubberized undercoating?
Definitely. The stuff is pretty durable. The same as any paint or undercoating, what you put it on has to be CLEAN, and scuffed so the product sticks.
Like 49 3100, my plan is to use spray-on bedliner for the running boards. They will be blasted and primered with epoxy first. Also considering it for the underside of the fenders to protect from rock chips.