Questions like in the title are difficult to formalize and discuss. The talk aims at trying to make it a bit more technical. We will discuss the following common deficiencies of mathematical models appearing in the MDE literature: missing constraints, missing structure, the very meaning of “something X is missing from a model”, and interconnections between constraints and structure. A specific context in which the issue in the title becomes technologically important is the quality of metamodels used in MDE practice and theory.