Returning JPA Entities in Rest Api's?

For quite simple apps, returning such entities works just fine and gets you started really quick. You are likely to need those for the database interactions anyway and the returned entities are usually 1:1 mapping to database.

For more robust applications which you want to scale better, completely circumventing those entities on reads by using plain SQL queries and DTOs is generally the more favourable approach.

For the write side, it's sometimes also better to use some form of a command pattern in your application (controller) layer, such as RegisterUser DTO, and only convert the command to actual JPA entity in your service layer.

I have worked in large web applications using the JavaEE stack and this was perfectly acceptable. You could use these entities in the JSP layer, the Beans layer, even for other stuff such as validations etc. This made your life easier and kept the code simple and clean. The overall solution still adhered to the JavaEE architecture.

However, when you are using a similar approach but outside of JavaEE, it can throw up problems that you otherwise would not encounter. The behavior of these entity classes outside JavaEE container, example in Spring or any other framework for example would be undefined. I always found using plain POJO or DTOs or JSONs much more beneficial in these cases.