Congratulations for comparing your backend options and for getting into Elixir / Phoenix.

I would say there is a significant upsides to a Phoenix backend, when going beyond a proof of concept / learning project. It is important to remember that an application with a real business behind it will face evolving requirements. Those imply in evolving the software.

When it gets to evolving the database schema over the lifetime of a software the differences between Phoenix with its plain relational database (often Postgres) and Firebase will show. They call Firebase a ‘schemaless’ database, right? In reality even a ‘schemaless’ DB has an implicit often even heterogeneous schema, as data has some form. When your changing business requirements demand that you migrate your persisted data to a new shape (schema) you are well covered by those tools the relational database you use with Phoenix offers. Firebase has no efficient solution to migrate the shape of thousands or millions data entries. So you will notice that you are trapped. It will be very difficult to evolve your application as business demands.

And even beyond data persistence, you are better positioned with Phoenix for a long lived project who’s code evolves. Even having serverless functions on Firebase for business logic, with Phoenix you have a cohesive package, full of tools like unit and integration tests. To keep complexity and correctness under control as your app evolves. A Firebase backend is more cobbled together. Cobbeling many simple services together is not simple, especially when maintaining the product on the long run.

In the end it all depends. If your project is a quick proof of concept, that does not back a business or you have no goal to evolve it, Firebase can be a good solution. But, thinking about professional development on a job or with an ambitious side project, you would probably choose a solution that makes software evolution productive and low risk.

I hope my answer contributed to your understanding. What do you think about those questions? How would you choose when you imagine yourself creating a product that backs a business?