Typically only use it where there is rust that can't be removed or for patches with fiberglass. I painted the ugly half of my rear bumper ~18 years ago when it was completely covered in rust. I used the marine clean and metal prep and two coats (always a 2nd coat). It has a uniform scattering of tiny rust spots, but overall it's solid and looks decent. I store it outside in winter and it doesn't see salt. I've also fixed small rust holes in the bottom with POR putty and paint. 20 years later they are still bullet proof.
POR has a consistency more like plastic than paint. You can't very well sand it, stick anything to it, or get anything apart that's been stuck together with it. That might be why body guys don't like to see it. Also, since it's so tough, rust could possibly develop under it. I use it for repairs, but keep it away from nice clean metal and use something else whenever possible.