Thank you for your reply.
I have no doubt that IntelliJ is a very good solution for developing with Scala. The point is that the Scala code is only one part of the whole which includes code generation for UML model and model-to-model transformations. This requires additional tools that I have to connect and to manage together as an integrated “tool chain”. This is where Eclipse has no competitor today, as far as I know.
Regarding the issues with the compiler, the point is just to integrate one - together with the build tool - in Eclipse. As I explain in my initial message I’m not an Eclipse specialist. With Scala-IDE you just had to install the plugin and everything was working. Without it, I have to do it “manually” but I don’t know how to proceed.