A two-stage Rotman lens based millimeter wave (mmWave) hybrid beamforming architecture for multiuser multiple-input multiple-output (MU-MIMO) operation is investigated. We identify a set of non-trivial losses within the complex analog phase-shifter network with the aid of a subscaled beamformer prototype, consisting of 15 element uniform rectangular array (URA) and two stage phase-shifter with 6 stacked Rotman lenses. The prototype is designed to operate at 28 GHz and is capable of 9 beam projections in azimuth and elevation zones. We evaluate the ergodic sum spectral efficiency of a 9-user uplink MU-MIMO system when the user equipments’ (UE) dominant paths are assumed to be aligned with the predefined beam directions. We point out the primary sources of some of the most prominent EM losses and discuss design guidelines for their possible mitigation at both the RF front-end and the signal processing (SP) blocks. Our investigation shows the practical limitations of the system and predicts the achievable performance. Interestingly, our conclusions hold valid for a scaled version of the same hybrid beamformer architecture supporting a larger number of antenna elements.