Sounds like an application issue.
I have a cerakoted shotgun that I use in a salt Marsh, the metal parts have never even scratched, even tried scratching it with a brass key, (the brass line wipes off) the wood has had some wear around the grip but I just painted right over it and it looks awesome.
Application can be an issue for sure. I assume that Cerakote is not nearly as uniform in thickness as proper anodizing and the quality is very much dependent on the applicator. But I am also willing to bet that your shotgun was steel and not aluminum, but I could be wrong. The issue with aluminum and Cerakote is that it is much softer than steel, dents easily and then the Cerakote can flake. Anodizing hardens the aluminum and from what I am reading is still the preferred as a precursor to Cerekote if a company chooses to Cerakote aluminum since it can bond well to the anodized surface.
I wonder if we could get unfinished sets for anywhere near the $90 that another company has advertised for their AR upper/lower combo?
I would go that route and outsource the anodizing myself but I doubt their production costs are that low.