I agree. I'd rather not explicitly state which side of the political spectrum I've noticed being a problem, but I've certainly seen a particular tendency to see articles that they politically oppose be much less neutral, and the references allowed in the first place very skewed towards one side.

I wonder to what extent it is that perhaps Wikipedia draws editors of a certain leaning (much as some professions seem to - on both sides).

Whatever, it's incredibly frustrating when you see the framing and feel the umpires are also often in the same bubble as the editor pushing their position on the article.