Comment by jpalomaki

I agree. If it sounds simple, then you are probably not thinking hard enough.

Think for example how to do backups. If you have a database and completely separate queue system your application state is distributed into two places. This means taking consistent backups is not straightforward. You can of course try to work around this on the application level (database as single source of truth, queues as "nice to have" -thing, but this makes things more complicated for the app).