I don't have a firm opinion on Cm (S), but Cm (O) are definitely overcosted. There's no way they're in general more useful than Irr LH (O) who have the same factors, far better manoeuvrability, and less tendency to be destroyed by foot. Sure, they scare Cv and Kn a bit, but at 2:3 they're likely to lose anyway, and the dune trick is quite situational.