Used sparingly, additives do work to help maintain a vehicle. As an example, I used Slick 50 in equipment at work back in the 1980s. It greatly improved the performance when I started using it in my vehicles. The problem was that other manufacturers did not use the same technic required to keep the teflon molecules in suspension, or people failed to read the instructions. Changes in manufacturing has resulted in my no longer needing to add it to my oil, but for a long time, it was a worthwhile additive.
The hard part is knowing what additives to use and how frequently to use it. Especially in this day of tight tolerances. I might try using seafoam on my older cars, but right now, all I do is a can of fuel injector cleaner twice a year.