Recent ab initio theoretical calculations of the electrical performance of several two-dimensional materials predict a low-field carrier mobility that spans several orders of magnitude (from 26,000 to 35 cm^2 V^-1 s^-1, for example, for the hole mobility in monolayer phosphorene) depending on the physical approximations used. Given this state of uncertainty, we review critically the physical models employed, considering phosphorene, a group V material, as a specific example. We argue that the

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... e of the most accurate models results in a calculated performance that is at the disappointing lower-end of the predicted range. We also employ first-principles methods to study high-field transport characteristics in mono- and bi-layer phosphorene. For thin multi-layer phosphorene we confirm the most disappointing results, with a strongly anisotropic carrier mobility that does not exceed ∼ 30 cm^2 V^-1 s^-1 at 300 K for electrons along the armchair direction.