An open question in the search for ultra-high-energy cosmic-ray (UHECR) sources is whether they are few and prominent or if a large population of sources collectively contributes to the diffuse flux. Motivated by this question, we investigated whether the latest UHECR data are consistent with originating from a population of sources that exhibit substantial diversity in terms of the cosmic-ray spectra they produce. We found that the fit to the cosmic ray data requires sources that are intrinsically very similar in terms of the maximum energy reached. This is surprising, considering the sizeable astrophysical diversity exhibited by the candidate astrophysical source populations. One source class that may satisfy the constraints imposed by the UHECR observations is non-jetted active galactic nuclei. An increasing number of observations suggest that semi-relativistic winds, termed ultra-fast outflows, are present in a large fraction of these sources. In this talk, I will show that cosmic ray nuclei can be accelerated to ultra-high energies in such ultra-fast outflows. Ultra-fast outflows can, therefore, possibly account for a substantial fraction of the observed diffuse UHECR flux.