> Spring Boot is an opinionated way to configure Spring applications.

If by "configure" you mean "make arbitrary changes to the behaviour of". Spring Boot adds a bunch of spring-boot-specific stuff that can't be replicated in vanilla Spring and isn't supported for use outside Spring Boot (e.g. @ConditionalOnMissingBean and friends are explicitly not supposed to be used in non-boot Spring configurations). This is a long way away from just sugar for an ordinary Spring configuration.

> Which mistakes is it repeating?

- Huge and incomprehensible

- Components can refer to other components in ways that are completely invisible in the code

- No way to understand your application's behaviour by just looking at your code, because it can vary drastically depending on the things that are instantiated by the container at runtime. (In EJB this was container-provided services, in Spring Boot it's configurations that are automatically instantiated if they're present on the classpath, without the application ever referring to them at all)

- In practice applications depend on implementation details of the framework and cannot safely upgrade or migrate