Normally every project needs custom values saved on products or another entities. Shopware does offer by default Custom fields. It looks nice, but it can produce very fast issues.
Before we start: This is my opinion
Let’s start with a simple Pro Contra list.
Pro  Can be easily created in the Administration / Code. Can define field types, labels etc and Shopware does the rendering in Administration for me Loaded by default on the entities Custom Fields are schema less.

Shyim's Brain

Custom fields in Shopware offer convenience for storing arbitrary data on entities, but come with significant drawbacks. Being schema-less and stored as JSON creates performance issues with SQL filtering and aggregation, causes Elasticsearch indexing problems due to dynamic type mapping, and exposes all fields publicly through APIs. The lack of schema also prevents proper associations, makes SQL modifications difficult, and creates inconsistent behavior between MySQL and MariaDB. Entity extensions are recommended as a better alternative despite requiring more initial setup effort.

Why I don't like Custom fields?

But when search / filtering / aggregation is slow. Can I not just use Elasticsearch? #

But Entity Extensions are so annoying to create #