What makes expert opinions unreliable?

There is a controversy on how to assess reliability and what makes something reliable or unreliable, as well as the significance of this, and this is due to the experts expressing ideas outside of their field, wandering and experimenting beyond the barriers of their level of expertise, or communicating these ideas in the form of a statistic with no scientific support or backing. There seems to be the need for a criteria that determines what will deem an opinion reliable or not, worth considering or merely an attempt to serve the views of the party.

This problem is most prominent in cases involving specific forms of forensic evidence that rely heavily on comparative and thus biased interpretation. Fingerprint and footprint analyses, handwriting, ballistics, fibres, or hair are examples of these pieces of evidence.