Self-registration has a lot of problems.
Global state, like you say - you can't merge two apps that use the same registration mechanism but use incompatible registered sets.
Lack of discoverability in maintenance - it's a kind of COME FROM but for data.
A barried to optimization: it's not clear what will break when you remove a dependency, and you have to eagerly run all initializers everywhere - if you try to be clever and lazy, you have to pick and choose, and you're back to explicit but indirect registration.
Initialization order problems: if you have code you run during init which depends on the stuff you register at init time, you're going to have to manually manage your initialization in error-prone ways. Adding or removing dependencies may change initialization order.
