  We show that within tachyonic braneworld models,"super-exponential" warp
factors of the form $e^{-2f} \sim e^{-2c_1e^{c_2 |\sigma|}}$ are problematic
when dealing with both the finiteness of the effective four-dimensional (4d)
Planck mass and the localization of 4d gravity, which can be stated by the
requirement that $\int e^{-2f(\sigma)}d\sigma < \infty$, because this condition
necessarily implies that c_1 and c_2 should be positive. As a consequence of
this fact the tachyonic field $T$ turns out to be complex in contradiction with
the real nature of the starting action for the tachyonic braneworld. Conversely
if one requires to have a real tachyon field, 4d gravity will not be localized
and the effective gravitational coupling will be infinite. We present several
typical examples where this problem occurs: we have analysed this situation for
thin as well as thick tachyonic braneworlds with 4d Poincare symmetry, for the
case when a bulk cosmological constant is present, and even for a brane with an
induced spatially flat 4d cosmological background, and shown that in all cases
the tachyon field T comes out to be inconsistently complex when imposing
localization of 4d gravity. On the other hand, when dealing with a further
reduction of the hierarchy problem one should carefully consider the sign of
the constants c_1 and c_2 to avoid inconsistencies in the tachyonic braneworld
model. We also present a similar discusion involving a canonical scalar field
in the bulk where none of these problems arise and hence, the mass hierarchy
and 4d gravity localization problems can be successfully addressed at once.
Finally, the stability analysis of this scalar tensor braneworld model with a
super-exponential warp factor is performed.
