Odoo Studio is best for:
- low to medium level of complexity features
- customizations created by individuals
- rapid development
- users with a low level of technical skills (scripting)
- customizations that are mainly for a single Odoo database
Odoo Modules are best for:
- medium to high level complexity features
- customizations created by teams
- traditional development with source control, automated tests, user acceptance phase, etc
- users with a high level of technical skills (programming)
- customizations that easily transport between different Odoo databases
Note: both support Python scripting via custom fields and the automation framework.
Another way to think about this:
Odoo Studio is like using off the shelf Lego sets that you don't need to design from scratch that meet generic needs. They come with instructions and are not meant to be modified very much from the original design.
Odoo Modules are like using individual bricks carefully chosen for something tailored specifically to your needs. You write your own instructions and can modify everything and create something highly customized.
See also:
https://www.youtube.com/watch?v=AENTv9r84Ns
You can tell them that you noticed that recent customizations were done via Odoo Studio rather than custom modules. While Studio works for small tweaks, it makes future Python-based development, maintainability, and upgrades more difficult.
Since you anticipated Python customizations from the start, you’d like to discuss refactoring current changes into modules and ensuring future customizations follow a module-based approach.