Pages content best practice

I see in the doc and in a lot of themes that people use a YML file in /data in order to edit the content of a homepage or product page for exemple instead of using /content. What are the best practices here and +/- of both options (especially when you use forestry).

Not familiar with Forestry. You need to refer to their documentation.

With that said it is not mandatory to use a data file to add content to the homepage or a product page.
One can use an under the root of /content/ to add metadata and .Content that applies only to the homepage.

The recommended way to organize your content in your Hugo project would be to use Page Bundles since these make many things easy, like Image Processing for example.