The authors have previously shown that there is a fundamental connection between the composition-dependence of the conduction-band-edge energy and the n-type carrier scattering cross-section in dilute alloys, imposing general limits on the electron mobility in Ga(In)NAs alloys and heterostructures. A simple general expression is derived for scattering in the ultra-dilute regime, showing that the scattering rate is proportional to |dEc/dx|2, the square of the initial variation of the conduction-band-edge energy with alloy composition x. The mobility estimated in GaNxAs1-x using the two-level band-anti-crossing (BAC) model is of the right magnitude (∼1000 cm2/Vs), but still larger than typical experimental values. The effects of N clusters and inhomogeneous broadening of energy levels are considered, including the formation of N-N pairs (where a single Ga atom has two N neighbours) and more complex clusters. It is shown that such complexes play a major role in further limiting the mobility, giving values of 200-400 cm2/Vs, in close agreement with experiment. It is concluded that random alloy scattering, rather than film quality or other factors, dominates the carrier mobility in these materials.