Efforts to expand the ligandable proteome have explored stereochemically defined fragments equipped with photoreactive and clickable handles (photo-stereoprobes) for mapping reversible small molecule-protein interactions in living systems. Here, we investigate the impact of structural elaboration of fragment photo-stereoprobes on interactions with the proteome of human cancer cells. We find that even conservative increases in size and complexity have profound effects on protein binding for fragment photo-stereoprobes, leading to enhanced stereoselective protein interactions, while abrogating others. We use stereochemically matched competitor ligands to assess the extent of protein binding in human cells, leading to the discovery of high-stoichiometry ligands for proteins from different functional classes, including a structurally novel set of epoxide hydrolase inhibitors. Our findings support the broad ligandability potential of the human proteome while also highlighting challenges in converting reversible small molecule-protein interaction maps into more advanced chemical probes.