The complex can only really be described by a full MO diagram. There is a wealth of experimental evidence, especially electronic spectroscopy (d−d∗ transitions being the most obvious example), and magnetic properties, that is in accordance with the MO picture and not the hybridisation one. It is simply impossible to explain many of these phenomena using this sp3 model.
Lastly, hybridisation alone cannot explain whether a complex should be tetrahedral ([NiCl4]2−) or square planar ([Ni(CN)4]2−, or [PtCl4]2−). Generally the effect of the ligand, for example, is explained using the spectrochemical series. However, hybridisation cannot account for the position of ligands in the spectrochemical series.