Spring Boot pulls in countless dependencies without warning. It can generate classes without you asking it to. It will run code just because you added a dependency, even if you don't explicitly call it. A lot of functionality is action-at-a-distance activated through annotations, which could just as easily be simple method calls which are easier to trace and debug. Version updates are aggressive about making breaking changes, sometimes seemingly for aesthetic reasons.

It just adds a lot of complexity even if you don't explicitly opt in to it or need it.