the down side to powder coating in my rustbelt experience is the lack of
creep resistance.  although you can purchase creep resistant powder
coating supplies almost no one that i know of uses them.  take a look at
trailer hitches to see what i mean.  the powder coat comes off in sheets
after the rust creeps underneath and the pressure exerted between the
steel and the powder coat peels the powder coating off.  creep resistance
is a standard coatings industry test criteria and in my opinion is one of
the important factors in choosing a paint that will live in a salt spray
or other corrosive environment.

sure in perfect world a super hard powder coat is the best, its low VOC,
extremely cost effective to apply because of the absence of over spray,
and the thick build up hides surface imperfections in one pass.  but in my
world things get scratched, no matter how hard the coating is and once
that happens you want something that has creep resistance unless you plan
on stripping things down to bare metal and redoing things every couple of
years.  or if you garage your powder coated vehicle in the winter if your
locality uses road salt.

jonathan