Circumferentially-split diffuser assemblies utilized within compression systems, such as centrifugal and mixed-flow compression systems employed within gas turbine engines, are provided. In embodiments, the diffuser assembly includes flow passages, which extend through the diffuser assembly and which include diffuser flow passage sections. Diffuser airfoils are interspersed with the diffuser flow passage sections. The diffuser airfoils include inboard and outboard airfoil segments distributed around a diffuser assembly centerline. The inboard and outboard airfoil segments are contained in and, thus, defined by inner and outer annular diffuser structures, respectively. The outer annular diffuser structure circumscribes the inner annular diffuser structure. In certain cases, the inboard airfoil segments and at least a portion of inner annular diffuser structure are composed of a first material, while the outboard airfoil segments and at least a portion of outboard annular diffuser structure are composed of a second material different than the first material.