I'm no expert here, however, at the risk of turning this into a "the paint peeled off my Chrysler minivan because of e-coat" thread, I'll throw in some more info....
E-coating is a dipping process that uses primer paint to coat the metal. The primer used in the e-coating process has an inherent weakness -- it is not UV stable. Because of this, it is important to apply a UV stable primer on top of the e-coated panel before applying a top coat. If any of you are familiar with the peeling paint problems from certain US products of the 80's and early 90's, much of this was due to the automakers not applying this second coat of primer. The top coat did not filter the UV rays, allowing the ecoat to be exposed. The e-coat would degrade, allowing the top coat to peel.
Consequently, if a repair panel has been ecoated and then stored for a long period of time where it is exposed to UV (sunlight), the e-coat may have started to degrade.
Other potential problems with e-coat are the same as with any primer system. The part needs to be clean and free of contaminates, such as the oils used in the stamping process, before ecoating. Also, all the products used to paint a car need to be compatible with one another (mixing enamel and lacquer products would be a disaster, as a simple example).
I'm not trying to sell this stuff -- just following the paths of others who have had success with it on their cars.
Bill / 1977 Alfa Romeo 4C2000 / 2012 BMW S1000RR / 1975 BMW R90/6