The cylinder heads and intake manifold on the Chevy engine in my sedan are powder coated gloss black. No signs of problems even at the exhaust header flange. 60K miles and counting.
Like the man said, if the powder coater doesn't know his stuff or uses the wrong material you can have problems. I had a rear axle housing done and the stuff scuffed worse than a rattle can paint job. They made it right but I never went back to them.
There are companies that only do stuff like lawn furniture and fence posts. Don't take custom parts to them. Look for a professional who knows how to do the job we and custom car builders/restorers expect. Or, learn to do it yourself