A good framework lets you gracefully progress from 100% autoconfigured to 95% autoconfigured, 5% customized to 90% autoconfigured, and so on. It does this by working in a consistent way, where framework-provided defaults behave the same as custom implementations of the same thing, and you can use the same tools and techniques to understand what the framework is doing as you use to understand your own application.

None of that's true of Spring Boot. The autoconfigured stuff comes in in its own fashion that's hard to relate to your normal configurations, and it's encapsulated in such a way that as soon as you want to replace part of it you find you have to reimplement all of it.