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That's not true. Configuration is just another input to a system and the outputs can be checked. There is nothing difficult about testing configs that is not already difficult about testing code.

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No, configuration defines the system and is therefore significantly harder to test. Application code can be unit tested and run in staging against simulated real world traffic. Configuration, on the other hand, often differs so much between production and staging that the only real way to test is to roll it out gradually and monitor the results. Just make staging and production identical then right? Easier said than done when your production infrastructure runs most of the worlds internet traffic. There’s a good reason that the last major outages to both AWS and Google were caused by infrastructure configuration changes, and it isn’t that their engineering sucks.

There is: many configuration changes are, by nature, global.

Code changes can be isolated and unit tested. Config changes often can't be.

You can still canary them, usually, but you lose some protection.

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