The coupling of coastal wave models, such as Boussinesq-type (BT) and Saint-Venant (SV) equations, has been explored since the 1990s. Despite numerous models and coupling examples, the literature exhibits significant disagreement regarding induced artifacts and methods for their analysis. This work aims to elucidate these issues, proposing explanations and a method for evaluating and comparing coupling techniques. We ground our explanation in the mathematical properties of each model's Cauchy and half-line problems, highlighting the sensitivity of these models to numerical artifacts. Additionally, we demonstrate how one-way models provide insights into expected physical effects, unexpected artifacts, and errors relative to 3D models. We demonstrate this analysis with linearized models, where we establish the well-posedness of a popular coupling, characterize analytically the "coupling error" in terms of wave reflections, and prove its asymptotic behavior in shallow water. We will discuss how these insights can be applied to other linear/nonlinear models, providing a foundation for the evaluation and comparison of new coupled coastal wave models.