Two dominant factors limiting efficiency of optical spin detection in ZnO-based materials system are identified from time-resolved optical orientation and magneto-optical studies. The first is related to the fundamental band structure of the materials characterized by a weak spin-orbit interaction. It leads to cancellation of circular polarization from the optical transitions between the conduction band and the A and B valence band states, which would otherwise carry the desired information on spin polarization of carriers. The second limiting factor is shown to be efficient carrier/exciton spin relaxation, i.e., about 45-80 ps.