Your question is too broad to be answered sufficiently. Please break down the question to specific issues that can be answered.
The first sentence of your question needs to be approached carefully. Most inventions (and hence most patents) are improvement upon others. So it's possible for anyone or any media to question the merit of any patent by its "degree of innovation", which is a rather subjective question. Also, every innovation is obvious in hindsight. An often used analogy is a safety pin, which is simple and obvious to anyone who has seen one, but most people won't have the wit to invent one without prior knowledge.
One objective of expert opinion, therefore, is to establish if all the elements/improvements of a patent are known and obvious to a person with ordinary skills in the art AT THE TIME of the invention. Thus, an expert must look for publicly accessible information that predates the patent application, and determine if the invention was anticipated by these information. A thorough knowledge of the history and technical details of the manufacturing process is generally not needed to understand an expert opinion.