  For the last few decades, one-nucleon knockout reactions on light composite
targets -- $^9$Be or $^{12}$C -- have been extensively used to study the
single-particle (s.p.) structure of nuclei far from stability. To determine
which information can be accurately inferred from knockout cross sections, we
conduct a sensitivity analysis of these observables considering various s.p.
descriptions within the usual eikonal description of the reaction. This work
shows that total one-neutron knockout cross sections are not sensitive to the
short-range part of the s.p. wave function. Rather, they scale with the mean
square radius of the overlap function. Using a perturbative expression of the
cross section, we can easily explain our numerical predictions analytically.
This analysis suggests that (i) spectroscopic factors extracted from knockout
data suffer from sizeable model uncertainties associated with the choice of
s.p. wave functions and (ii) knockout reactions constitute an excellent probe
of the radius of the nucleus and therefore offer an alternative technique to
infer the neutron-skin thickness of exotic nuclei.
