The long-wavelength stability is analyzed for a surfactant-laden, viscoelastic liquid flowing down an inclined plane when the liquid undergoes additional interfacial shear. The upper convected Maxwell model is employed for describing the elastic nature of the fluid. The system stability is characterized by the interface and the surfactant modes. The interface mode involves both elastic and Marangoni effects that modulate the stability with applied shears and gravity-driven flow. The surfactant mode is only determined by the shear-induced Marangoni effects. A phase diagram is established to identify the dominant mode and the overall features of instability. It reveals that the system is susceptible to instability, except in a stable window when the applied shear opposes the gravity-driven flow.