A Sufi order authenticates itself by its silsila, the chain of transmission running back from the living master through his master and so on. An extraordinary proportion of those chains, across orders that agree on very little else, pass through Junayd. He is the point at which the tradition consolidated: the teacher whose formulations were careful enough to be quotable by people who needed to demonstrate that they were not saying anything actionable.
That caution was earned. He taught in Baghdad in the decades when the relation between Sufi practice and the jurists was being negotiated and had not yet been settled, and much of his surviving work is in letters, written in a compressed and deliberately difficult idiom, meant for people who already knew what he was talking about. He is said to have warned al-Hallaj, who had been in his circle, that his openness would end badly, and the tradition preserves an account in which Junayd was asked to sign the legal opinion against him and answered on the basis of the outward law rather than the inward state.
The long consequence is that Sufism had a respectable face available when it needed one. When al-Ghazali, two centuries later, argued that Sufi practice belonged inside mainstream Sunni religion rather than outside it, the material he was defending was largely of this school. Whether that respectability was a preservation or a domestication is a question the tradition itself has argued about ever since.