  We evaluate in the framework of QCD factorization the two-loop vertex
corrections to the decays $\bar{B}_{(s)}\to D_{(s)}^{(\ast)+} \, L^-$ and
$\Lambda_b \to \Lambda_c^+ \, L^-$, where $L$ is a light meson from the set
$\{\pi,\rho,K^{(\ast)},a_1\}$. These decays are paradigms of the QCD
factorization approach since only the colour-allowed tree amplitude contributes
at leading power. Hence they are sensitive to the size of power corrections
once their leading-power perturbative expansion is under control. Here we
compute the two-loop ${\cal O}(\alpha_s^2)$ correction to the leading-power
hard scattering kernels, and give the results for the convoluted kernels almost
completely analytically. Our newly computed contribution amounts to a positive
shift of the magnitude of the tree amplitude by $\sim 2$\%. We then perform an
extensive phenomenological analysis to NNLO in QCD factorization, using the
most recent values for non-perturbative input parameters. Given the fact that
the NNLO perturbative correction and updated values for form factors increase
the theory prediction for branching ratios, while experimental central values
have at the same time decreased, we reanalyze the role and potential size of
power corrections by means of appropriately chosen ratios of decay channels.
