In the context of digital advertising / paid search, what are typical disadvantages of multivariate testing?

In the context of digital advertising / paid search, what are typical disadvantages of multivariate testing? Answers must include: Definition of terminology, Importance of the concept or process, and the Process of how a professional marketer approaches the practice. Please start your answer so it can stand alone as a sentence/paragraph. For example, “SEO keyword analysis is the process of…”