  Fermions scattering on a black hole background cannot develop an instability
sourced by superradiance. However, in a global (or planar)
AdS$_4$-Reissner-Nordstr\"om background fermions can violate the AdS$_2$
fermionic mass stability bound as measured by a near horizon observer at zero
temperature. This suggests that AdS-Reissner-Nordstr\"om black holes might
still be unstable to Dirac perturbations. Motivated by this observation we
search for linear mode instabilities of Dirac fields in these backgrounds but
find none. This is in contrast with the scalar field case, where a violation of
the near-horizon Breitenl\"ohner-Freedman stability bound in the
AdS-Reissner-Nordstr\"om background triggers the already known scalar
condensation near-horizon linear instability (in the planar limit this is
Gubser's instability that initiated the holographic superconductor programme).
We consider both the standard and alternative AdS/CFT quantizations (that
preserve the conformal invariance of AdS). These are reflective boundary
conditions that have vanishing energy flux at the asymptotic boundary.
