This talk provides a brief overview of (in)stability results, spanning from the massless wave equation to nonlinear perturbations of extremal Reissner–Nordström spacetime. We then focus on the linear perturbation theory, including results for gauge-invariant quantities such as the Teukolsky variables. Building on these results, we address ongoing developments in establishing boundedness for gauge-dependent quantities. If time permits, we explore mechanisms that yield only boundedness in the extremal case, contrasting this with sub-extremality, where decay is also present.