  We study class $\mathcal S$ for locally compact groups. We characterize
locally compact groups in this class as groups having an amenable action on a
boundary that is small at infinity, generalizing a theorem of Ozawa. Using this
characterization, we provide new examples of groups in class $\mathcal S$ and
prove unique prime factorization results for group von Neumann algebras of
products of locally compact groups in this class. We also prove that class
$\mathcal S$ is a measure equivalence invariant.
