> but even then, an expert's opinion is only more valid than a layman's because of specialized knowledge or experience; either learned from another expert, or observed directly

Another problem: "an expert's opinion" can be considered from various perspectives, like on average, or also on a per opinion basis - and, one can (at least in theory) take complexity into consideration (say: multi-variate causality), or ignore it (and thus perceive that it does not exist).

> If a so called "expert" made an assertion without presenting citation or evidence, I think it's perfectly reasonable to challenge that assertion.

I think it's perfectly reasonable to challenge all "expert" assertions, though doing so skilfully is not our strong suit.