Odoo Studio (Enterprise) lets you customize models, views, and automations visually without writing code. It's powerful but has limitations. A custom module gives full control but requires developer knowledge. The question is: where's the boundary?
Studio works well for
Adding a field to an existing view. Changing layout on a form. Creating a simple automation rule. Changing labels in the interface. Adding a step to a pipeline. Everything that's configuration rather than logic.
Studio saves changes in the database as ir.model.fields and ir.ui.view records. It works, but it's harder to version control and move between environments (staging → production) compared to a module in Git.
Custom module is needed for
Complex business logic (calculations, validations, workflows). Integrations with external systems. New reports. Custom controllers (web endpoints). Everything requiring Python code, security configuration, or data migration.
Hybrid approach
Use Studio for quick prototyping. When the client approves, rewrite as a module. The module can be version-controlled, automatically tested, and deployed in a controlled manner. Studio changes can be exported as a module, but the result is often harder to maintain than handwritten code.
Our recommendation
Small customizations the client makes themselves: Studio. Everything to be maintained long-term by a developer: custom module. And if you're on CE: Studio doesn't exist, so it's custom module all the way. But it doesn't have to be complicated. A module with one field and one view change is ten lines of code.