My experience with POR 15 is just the opposite. I've used it on frames and engines and find it to be durable and tough. Sprayed the frame on my pickup last year and there is no shrinkage and no chips. I can't even chip it when I hit it with a hammer.
All that said, that doesn't mean its the best stuff out there. POR is not the latest technology and does have some problems when exposed to direct sunlight (doesn't hurt it's rust inhibiting capabilities but just chalks the color) and it does not hold up well when exposed to salt over long periods of time. Other than that it works very well.