I've never seen any cracking or degrading of ceramic coatings in the 3 different exhaust applications I've used it on. 1 is my Olds Cutlass over stock manifolds which is now 10-12 years old, 1 was on my Skyline and that routinely saw temps of 600C+ for 5-6 years, and the last is the Doug Thorley header on the Jeep. The first 2 of those were parked 50ft from the ocean every night, but never saw off-road obviously. The Jeep has only had the coated header for like 2 years so not a good test. Worst case I could imagine is you have a few spots the coating is thin and you get a bit of surface rust, plus the thermal protection at that spot is lower.
I think any concerns about them rusting is unfounded. Not saying the coating can't fail, flake off, or even allow a little rust to happen where it's thin. A number of factors including improper prep, application, or other things could probably affect it. As far as micro cracks allowing corrosion to get under the coating and make it rust worse that an uncoated header, that's just a false idea not based in reality.
If you aren't concerned about looks, I'd consider Swain tech ceramic coating. That stuff is supposed to be the best coating for insulation and is stupid durable. It's just fugly IMHO. Do that plus the inconel heat shield!! Might cost about as much as your motor does though lol.