Any examples of trivial things you've struggled with?

My Spring Boot experience is the opposite and it is usually my go-to framework when I need to get a backend up and running fast. It takes 30-50 lines of Kotlin to serve some JSON endpoints backed by a SQL database. And those lines are mostly defining the shape of the JSON response, database model, and hooking up the request route to the controller and repository. All the rest is generated, inferred or in libraries.

Everything?

It's all very opaque and specific to Spring; and it keeps changing every few months, which means that a lot of information online is not applicable.

I strongly prefer writing my own scaffolding and pulling in specialized libraries for specific tasks.

Same. I've tried other frameworks like Django and even Express, but I still go back to Kotlin + Spring when I need a quick backend for a personal project.