The solution of large sparse linear systems is one of the most important kernels in many numerical simulations. The domain decomposition methods (DDM) community has developed many efficient and robust solvers in the last decades. While many of these solvers fall in Abstract Schwarz (AS) framework, their robustness has often been demonstrated on a case-by-case basis. In this paper, we propose a bound for the condition number of all deflated AS methods provided that the coarse grid consists of the assembly of local components that contain the kernel of some local operators. We show that classical results from the literature on particular instances of AS methods can be retrieved from this bound. We then show that such a coarse grid correction can be explicitly obtained algebraically via generalized eigenproblems, leading to a condition number independent of the number of domains. This result can be readily applied to retrieve the bounds previously obtained via generalized eigenproblems in the particular cases of Neumann-Neumann (NN), additive Schwarz (aS) and optimized Robin but also generalizes them when applied with approximate local solvers. Interestingly, the proposed methodology turns out to be a comparison of the considered particular AS method with generalized versions of both NN and aS for tackling the lower and upper part of the spectrum, respectively. We furthermore show that the application of the considered grid corrections in an additive fashion is robust in the aS case although it is not robust for AS methods in general. In particular, the proposed framework allows for ensuring the robustness of the aS method applied on the Schur complement (aS/S), either with deflation or additively, and with the freedom of relying on an approximate local Schur complement, leading to a new powerful and versatile substructuring method. Numerical experiments illustrate these statements.