  We present growth of structure constraints from the cosmological analysis of
the power spectrum multipoles of SDSS-III BOSS DR12 galaxies. We use the galaxy
power spectrum model of Hand et al. (2017), which decomposes the galaxies into
halo mass bins, each of which is modeled separately using the relations between
halo biases and halo mass. The model combines Eulerian perturbation theory and
halo model calibrated on $N$-body simulations to model the halo clustering. In
this work, we also generate the covariance matrix by combining the analytic
disconnected part with the empirical connected part: we smooth the connected
component by selecting a few principal components and show that it achieves
good agreement with the mock covariance. Our analysis differs from recent
analyses in that we constrain a single parameter $f\sigma_8$ fixing everything
else to Planck+BAO prior, thereby reducing the effects of prior volume and
mismodeling. We find tight constraints on $f\sigma_8$:
$f\sigma_8(z_{\mathrm{eff}}=0.38)=0.489 \pm 0.038$ and
$f\sigma_8(z_{\mathrm{eff}}=0.61)=0.455 \pm 0.028$ at $k_{\mathrm{max}} = 0.2\
h$Mpc$^{-1}$, with an overall amplitude error of 5%, and in good agreement
(within 0.3 sigma) of Planck amplitude. We discuss the sensitivity of
cosmological parameter estimation to the choice of scale cuts, covariance
matrix, and the inclusion of hexadecapole $P_4(k)$. We show that with
$k_{\mathrm{max}} = 0.4\ h$Mpc$^{-1}$ the constraints improve considerably to
an overall 3.2% amplitude error, but there is some evidence of model
misspecification on MultiDark-PATCHY mocks. Choosing $k_{\mathrm{max}}$
consistently and reliably remains the main challenge of RSD analysis methods.
