As far as I am aware, combining powers has always led to oppression. You have left-wing communists, and right-wing fascists both attempt to combine the powers of government and economic production and fail. This also seems to hold true in the past when the issue was more the state and the church - separating these entities was of paramount importance to establishing some manner of liberty in the U.S. We took this idea even further and separated the government powers into 3 branches, and even delegated some powers to the states, counties, and cities.

I am unaware of any historical examples where combining powers like this in one body has not led to more rather than less authoritarian conditions, though perhaps some exceptions to this rule exist, as I am not the worlds best historian by any means