A comparative investigation of shear-layer instability mechanisms in wall-bounded flows is performed by employing both spatial and temporal direct numerical simulations. In this study, two complex flows are considered, the transitional backward-facing step flow and the turbulent wall-jet over plane and curved surfaces (Coanda flow). For the temporal simulations, (mean) profiles that are obtained from the spatial simulations are used as base flows. Temporal simulations neglect any spatial evolution of both the base flow and the disturbances and can therefore be used to investigate the local stability behavior. First the primary Kelvin-Helmholtz type instability of the shear layer is addressed. For secondary instabilities (subharmonic and fundamental), the investigation aims to determine the dominant instability mechanisms responsible for the occurrence of the typical flow structures that are observed in the spatial simulations. The complimentary nature of the temporal simulations provides crucial information for the interpretation of the spatial simulation results.