In this paper we investigate the long-term stability and reliability of a 3D-Opto-MID package for multimode waveguide bus coupling. Preliminary works proved an interruption-free coupling approach with asymmetric coupling rates depending on the coupling direction for optical bus systems. This is possible thanks to a coupling at the side-faces of the waveguide cores where one coupling partner has a defined bending to achieve the asymmetric coupling. Crucial for this defined bending are three-dimensional assemblies. For this, we developed a mixed-material package based on ceramic thick-film substrates combined with stereo-lithographic printed polymers. Because of the different material parameters, we analyzed the package behavior in a high temperature storage, a temperature humidity and a temperature cycle test. The results of the reliability tests enable specific further development of the package and thus make an important contribution to the intended series production of the 3D-Opto-MID technology.