The m * σ and * S can be used to calculate the Fermi surface complexity factor (N * V K * ), where N * V is the effective valley degeneracy, i.e. the number of band extrema involved in the transport, and K * is the effective anisotropy factor that describes the shape of the carrier pockets in the Fermi surface. The (N * V K * ) shows a strong correlation to the maximum power factor, and values above ∼10 are characteristic of materials such as the lead chalcogenides where features of the electronic structure are known to contribute significantly to the thermoelectric performance. We calculate (N * V K * ) = 4.6/5.1 for p-type Pnma SnS/SnSe and 8.2/11 for the n-type materials, while the larger m * σ of the π phases result in 60-80% smaller values of 1.1/1.7 and 3.2/3.7. This implies that the electronic structures of the Pnma phases show features that are beneficial for the thermoelectric properties, and that these are largely absent in the π phases. While we do not calculate the N V or K, the values of (N * V K * ) = 1.1-3.7 predicted for the π phases suggest either a low effective valley degeneracy and/or an effective anisotropy K < 1.