This paper presents a novel unified performance analysis of Space-Time Block Codes (STBCs) operating in the Multiple Input Multiple Output (MIMO) network and subjected to generalized shadowed fading and noise scenarios. Specifically, we derive novel, simple and accurate average bit error rates (ABER) expressions for coherent modulation schemes in the generalized η-μ and shadowed κ-μ fading channels. The noise in the network is assumed to be modeled by the additive white generalized Gaussian noise (AWGGN), which encompasses the Laplacian and the Gaussian noise environments as special cases. The result obviates the need to re-derive the error rates for MIMO STBC systems under many multipath fading and noise models, while avoiding computationally-expensive expressions. Published results from the literature as well as numerical simulations corroborate the accuracy of our derived generalized expressions.