Some of the pains with Spring Boot don't go away even when you understand it better. The fact that the framework relies so heavily on reflection makes debugging a pain, turns what should be compile-time errors into runtime errors (which, among other things, makes updating libraries annoying), and also leads to very slow integration tests. I also haven't yet met a developer who fully understands what @Transactional does, exactly.

I agree Spring Boot brings a lot to the table (Spring Security alone, for example), but at the cost of significant downsides.

I see the benefit for larger and more diverse teams, but personally I would choose lighter-weight frameworks and libraries, even if at the cost of having to do more plumbing myself.