Jung's engagement with esoteric material was neither a hobby nor a late aberration; it was where he looked for evidence. His argument required that certain symbolic structures recur in people who could not have learned them from each other, and alchemy gave him a large, obscure, historically remote body of imagery to compare against the dreams and fantasies of patients who had never read any of it.
His reading of it is a reading, and its status should be clear. Jung treated the alchemical opus as a projection of the individuation process, the nigredo as psychological dissolution, the coniunctio as integration of opposites, the philosopher's stone as the realised Self. This was hugely productive psychologically and it is contested historically. Lawrence Principe and William Newman have argued in detail that the alchemists were doing laboratory chemistry, that their allegorical language often encodes specific material operations, and that the wholly spiritual reading of alchemy is largely a nineteenth-century invention that Jung inherited and made permanent. Both things can be true: Jung may have been wrong about what the alchemists were doing and right that their imagery maps something real about psychological change.
The Red Book, the record of his own visionary experiments between 1913 and 1917, was withheld from publication until 2009 and reframed him when it appeared, it shows the concepts arriving through active imagination and personal crisis rather than through clinical observation.
His conduct in the 1930s remains genuinely disputed rather than settled. He took the presidency of the reorganised international psychotherapy society under German influence in 1933 and published remarks distinguishing Jewish from Germanic psychology that were antisemitic in effect and were read as such at the time. He later called his position a misjudgement. Whether it was accommodation, opportunism, or naivety about what he had joined is argued in the literature and no consensus exists.