Powder coating when properly done is a superior product over the typical spray job, but with any coating the key is preparation and application. Usually when a coating of any type doesn't adhere correctly, the problem is either a contaminated surface (like oil on tubing) or the weather conditions are bad (humidity to high). Many trailer manufacturers that spray don't get a good coating on the bottom of the trailer and that is where it needs it the most. Powder coating when done correctly gives and even coat over every bit of exposed surface and gets paint into every nook and cranny.
I have a Kearney trailer (27 +5) that was powder coated and has held up very well. Looks as good now as when I bought it 7 years ago. Now the boards they put on them are junk but thats another story.