> a message bus is kinda integrated in the BEAM runtime or Erlang

You have that on a single node. If you need to run more than one node, you will end up inventing your own on top of mnesia and usually the results are not spectacular or/and you will end up paying happihacking to do it for you. Or one of the other Erlang oldtimers who you can count on the fingers of your hands.

This is really suboptimal compared to what you can achieve by using any normal language + any message bus. You are actually much better using a proper message bus even if you use Erlang.