  We study systematics associated with estimating simple stellar population
(SSP) parameters -- age, metallicity [M/H], $\alpha$-enhancement [$\alpha$/Fe]
and IMF shape -- and associated $M_*/L$ gradients, of elliptical slow rotators
(E-SRs), fast rotators (E-FRs) and S0s from stacked spectra of galaxies in the
MaNGA survey. These systematics arise from (i) how one normalizes the spectra
when stacking; (ii) having to subtract emission before estimating absorption
line strengths; (iii) the decision to fit the whole spectrum or just a few
absorption lines; (iv) SSP model differences (e.g. isochrones, enrichment,
IMF). The MILES+Padova SSP models, fit to the H$_\beta$, $\langle$Fe$\rangle$,
TiO$_{\rm 2SDSS}$ and [MgFe] Lick indices in the stacks, indicate that out to
the half-light radius $R_e$: (a) ages are younger and [$\alpha$/Fe] values are
lower in the central regions but the opposite is true of [M/H]; (b) the IMF is
more bottom-heavy in the center, but is close to Kroupa beyond about $R_e/2$;
(c) this makes $M_*/L$ about $2\times$ larger in the central regions than
beyond $R_e/2$. While the models of Conroy et al. (2018) return similar [M/H]
and [$\alpha$/Fe] profiles, the age and (hence) $M_*/L$ profiles can differ
significantly even for solar abundances and a Kroupa IMF; different responses
to non-solar abundances and IMF parametrization further compound these
differences. There are clear (model independent) differences between E-SRs,
E-FRs and S0s: younger ages and less enhanced [$\alpha$/Fe] values suggest that
E-FRs and S0s are not SSPs, but relaxing this assumption is unlikely to change
their inferred $M_*/L$ gradients significantly.
