More and more efforts are directed towards the standardization of the methods of determining the functional unit (FU) in a Life Cycle Assessment (LCA). These efforts concern the development of theories and detailed methodological guides, but also the evaluation of the quality of the FU obtained. The objective of this article is to review this work in order to propose, using a multiscale approach, a method for defining the FU in the mining sector, which takes into account all the dimensions of the system under study. In this first part, the emphasis is on identifying the shortcomings of the FU. The absence of a precise normative framework specific to each sector of activity, as well as the complex, multifunctional and hard-to-scale nature of the systems concerned, are at the origin of the flexibility in the selection of the FU. This lack of a framework, beyond generating a heterogeneous definition of the FU for the same system, most often leads to an incomplete formulation of this sensitive concept of LCA. It has been found that key parameters such as the end-use of a product or process, as well as the interests of stakeholders, are hardly taken into account in the specification of the FU.