As someone who's liked using DI frameworks, I'm curious: if you do it manually, as your codebase gets large, doesn't it get harder to "plumb" a new object that needs to get used somewhere at a deep level? This seems like it'd get more unwieldy when refactoring.

I've never worked in a large codebase that did this manually so I'm not sure what it looks like. The large codebases I've seen that don't use DI have used something like a service locator, singletons, or constructed everything where it was needed (and used extensive mocking framework functionality for testing).