My Hinckley Trident is now 29 years & 102,000 miles old & the powder coating on the frame is as good as the day it left the Triumph works. Quality powders & proper surface prep. are the key. On the other hand I have seen TVR car chassis with the PC falling off in just a couple of years, which I know from a bloke who worked there as a welder was down to the chassis coming off the welding fixture, being wiped over with thinners & then coated.
A good paint job is only a good paint job with the same attention to detail. I always prefered stove enamel but it seems almost impossible to find any companies to do it these days.
Martyn.