Specifically, wikipedia is not an unbiased source on any issue that is of particular concern to the terminally online. Anything to do with US politics or culture war is going to be heavily distorted due to the nature of the beast (its something people care a lot about, there were strong founder effects and there was a purity spiral).

The sources that are allowed/disallowed and their respective biases (e.g. what they publish or don't say) are a big factor.